Difference between revisions of "KVM"

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[[it:KVM]]
 
[[it:KVM]]
 
[[zh-CN:KVM]]
 
[[zh-CN:KVM]]
{{Out of date|{{Pkg|qemu-kvm}} no longer exists as all of its features have been merged into {{Pkg|qemu}}. Whole page needs update.}}
+
{{Article summary start}}
{{Merge|QEMU|For the same reason as above, I suggest to merge as much as possible into [[QEMU]]. It's not clear if things in this article are KVM-specific or can be applied to plain QEMU too.}}
+
{{Article summary text|This article covers checking for KVM support and some KVM-specific notes, features etc. It does not cover features common to multiple emulators using KVM as a backend. You should see related articles for such information.}}
'''KVM''', Kernel-based Virtual Machine, is a hypervisor built right into the Linux kernel. It is similar to [[Xen]] in purpose but much simpler to get running. To start using the hypervisor, just load the appropriate {{Ic|kvm}} kernel modules and the hypervisor is up. As with Xen's full virtualization, in order for KVM to work, you must have a processor that supports Intel's VT-x extensions or AMD's AMD-V extensions.
+
{{Article summary heading|Related}}
 +
{{Article summary wiki|QEMU}}
 +
{{Article summary wiki|Libvirt}}
 +
{{Article summary wiki|VirtualBox}}
 +
{{Article summary wiki|Xen}}
 +
{{Article summary wiki|VMware}}
 +
{{Article summary end}}
 +
'''KVM''', Kernel-based Virtual Machine, is a hypervisor built into the Linux kernel. It is similar to [[Xen]] in purpose but much simpler to get running. Unlike native [[QEMU]], which uses emulation, KVM is a special operating mode of QEMU that uses CPU extensions ([[Wikipedia:Hardware-assisted virtualization|HVM]]) for virtualization via a kernel module. KVM originally supported x86 and x86_64 architectures and has been ported to S/390, PowerPC, IA-64, and ARM (since Linux 3.9).
  
Using KVM, one can run multiple virtual machines running unmodified GNU/Linux, Windows, or any other operating system. (See [http://www.linux-kvm.org/page/Guest_Support_Status Guest Support Status]). Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc. See [http://www.linux-kvm.org/page/HOWTO KVM Howto].
+
Using KVM, one can run multiple virtual machines running unmodified GNU/Linux, Windows, or any other operating system. (See [http://www.linux-kvm.org/page/Guest_Support_Status Guest Support Status] for more information.) Each virtual machine has private virtualized hardware: a network card, disk, graphics card, etc.
  
 
Differences among KVM, Xen, VMware, and QEMU can be found at the [http://www.linux-kvm.org/page/FAQ#General_KVM_information KVM FAQ].
 
Differences among KVM, Xen, VMware, and QEMU can be found at the [http://www.linux-kvm.org/page/FAQ#General_KVM_information KVM FAQ].
KVM is used alongside [[Libvirt]] for better management with "virsh" command set. Check [[Libvirt]] for more details.
 
  
== Get the packages ==
+
== Checking support for KVM ==
  
Arch Linux kernels provide the appropriate [[Kernel_modules|kernel modules]] to support KVM.
+
=== Hardware support ===
You can check if your kernel supports KVM with the following command (assuming your kernel is built with {{ic|CONFIG_IKCONFIG_PROC}}):
+
$ zgrep KVM /proc/config.gz
+
  
 
KVM requires that the virtual machine host's processor has virtualization support (named VT-x for Intel processors and AMD-V for AMD processors). You can check whether your processor supports hardware virtualization with the following command:
 
KVM requires that the virtual machine host's processor has virtualization support (named VT-x for Intel processors and AMD-V for AMD processors). You can check whether your processor supports hardware virtualization with the following command:
 
  $ lscpu
 
  $ lscpu
  
You processor supports virtualization only if there is a line telling you so.
+
Your processor supports virtualization only if there is a line telling you so.
  
 
You can also run:
 
You can also run:
 
  $ grep -E "(vmx|svm)" --color=always /proc/cpuinfo
 
  $ grep -E "(vmx|svm)" --color=always /proc/cpuinfo
  
If nothing is displayed after running that command, then your processor does ''not'' support hardware virtualization, and you will ''not'' be able to use KVM.
+
If nothing is displayed after running that command, then your processor does '''not''' support hardware virtualization, and you will '''not''' be able to use KVM.
  
== Set up kernel modules ==
+
=== Kernel support ===
  
First, you need to add your user account into the {{Ic|kvm}} group to use the {{ic|/dev/kvm}} device.
+
You can check if necessary modules ({{ic|kvm}} and one of {{ic|kvm_amd}}, {{ic|kvm_intel}}) are available in your kernel with the following command (assuming your kernel is built with {{ic|CONFIG_IKCONFIG_PROC}}):
  # gpasswd -a <Your_Login_Name> kvm
+
  $ zgrep KVM /proc/config.gz
  
{{Note|If you use systemd and are a local user, this is not necessary, as access is now granted by systemd/udev.}}
+
{{Note|Arch Linux kernels provide the appropriate [[Kernel_modules|kernel modules]] to support KVM.}}
  
Secondly, you have to choose one of the following depending on the manufacturer of the VM host's CPU.
+
=== Loading kernel modules ===
  
* If you have Intel's VT-x extensions, modprobe the {{Ic|kvm_intel}} module.
+
You need to load {{ic|kvm}} module and one of {{ic|kvm_amd}} and {{ic|kvm_intel}} depending on the manufacturer of the VM host's CPU. See [[Kernel modules#Loading]] and [[Kernel modules#Manual module handling]] for information about loading kernel modules.
# modprobe kvm_intel
+
 
+
* If you have AMD's AMD-V (code name "Pacifica") extensions, modprobe the {{Ic|kvm-amd}} module.
+
# modprobe kvm_amd
+
  
 
If modprobing {{Ic|kvm_intel}} or {{Ic|kvm_amd}} fails but modprobing {{Ic|kvm}} succeeds, (and {{ic|lscpu}} claims that hardware acceleration is supported), check your BIOS settings. Some vendors (especially laptop vendors) disable these processor extensions by default. To determine whether there's no hardware support or there is but the extensions are disabled in BIOS, the output from {{Ic|dmesg}} after having failed to modprobe will tell.
 
If modprobing {{Ic|kvm_intel}} or {{Ic|kvm_amd}} fails but modprobing {{Ic|kvm}} succeeds, (and {{ic|lscpu}} claims that hardware acceleration is supported), check your BIOS settings. Some vendors (especially laptop vendors) disable these processor extensions by default. To determine whether there's no hardware support or there is but the extensions are disabled in BIOS, the output from {{Ic|dmesg}} after having failed to modprobe will tell.
  
If you want these modules to persist, see [[Kernel_modules#Loading]].
+
{{Note|Newer versions of [[udev]] should load these modules automatically, so manual intervention is not required.}}
  
 
== How to use KVM ==
 
== How to use KVM ==
 +
See the main article: [[QEMU]].
  
As {{ic|qemu-kvm}} package is merged into {{Pkg|qemu}}, see the main article [[QEMU]], and especially the section [[QEMU#Enabling KVM|Enabling KVM]].
+
== Tips and tricks ==
  
== Paravirtualized guests (virtio) ==
+
{{Note|See [[QEMU#Tips and tricks]] and [[QEMU#Troubleshooting]] for general tips and tricks.}}
  
KVM offers guests the ability to use paravirtualized block and network devices, which leads to better performance and lower overhead.
+
=== Nested virtualization ===
For Windows, a paravirtualized network driver can be obtained [http://www.linux-kvm.org/page/WindowsGuestDrivers/Download_Drivers here].
+
FreeBSD has the ability to use virtio drivers since 10.0 (unreleased). A backport of the drivers are available in the port {{ic|emulators/virtio-kmod}} for FreeBSD 8.3 and 9.0.
+
  
A virtio block device requires the option {{Ic|-drive}} instead of the simple {{Ic|-hd*}} plus {{Ic|1=if=virtio}}:
+
{{Expansion|Is it possible also with {{ic|kvm_amd}}?}}
$ qemu-kvm -boot order=c -drive file=drive.img,if=virtio
+
  
{{Note|{{Ic|1=-boot order=c}} is absolutely necessary when you want to boot from it. There is no auto-detection as with {{Ic|-hd*}}.}}
+
On host, enable nested feature for {{ic|kvm_intel}}:
 +
# modprobe -r kvm_intel
 +
# modprobe kvm_intel nested=1
  
Almost the same goes for the network:
+
To make it permanent (see [[Kernel modules#Setting module options]]):
$ qemu-kvm -net nic,model=virtio
+
{{hc|/etc/modprobe.d/modprobe.conf|<nowiki>
 
+
options kvm_intel nested=1
=== Preparing an (Arch) Linux guest ===
+
 
+
To use virtio devices after an Arch Linux guest has been installed, the following modules can be loaded in the guest: {{Ic|virtio}}, {{Ic|virtio_pci}}, {{Ic|virtio_blk}}, {{Ic|virtio_net}}, and {{Ic|virtio_ring}} (for 32-bit guests, the specific "virtio" module is not necessary).
+
 
+
If you want to boot from a virtio disk, the initial ramdisk must be [[mkinitcpio|rebuilt]]. Add the appropriate modules in {{ic|/etc/mkinitcpio.conf}} like this:
+
MODULES="virtio_blk virtio_pci virtio_net"
+
and rebuild the initial ramdisk:
+
# mkinitcpio -p linux
+
 
+
Virtio disks are recognized with the prefix '''''v''''' (e.g. ''v''da, ''v''db, etc.); therefore, changes must be made in at least {{ic|/etc/fstab}} and {{ic|/boot/grub/menu.lst}} when booting from a virtio disk. When using grub-pc which references disks by [[Persistent_block_device_naming#By-uuid|UUID's]], nothing has to be done.
+
 
+
Edit or create {{ic|/boot/grub/device.map}}:
+
(hd0) /dev/vda
+
 
+
{{Note|The following may be outdated since official installation ISO 2011.08.19. The repositories now offer GRUB v2.}}
+
 
+
To enable virtio at Arch Linux installation time, manual GRUB installation is required (for arch-release-media 2010.05).
+
Though AIF correctly detects the virtio disks and sets up the right prefixes, the {{ic|/boot/grub/device.map}} file must be created before configuring the bootloader.
+
 
+
So when installing Arch Linux, you can install GRUB by switching to another virtual terminal ({{Keypress|Ctrl+Alt+F2}}) and running the following commands.
+
# grub
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> device (hd0) /dev/vda
+
> root (hd0,0)
+
> setup (hd0)
+
> quit
+
 
+
{{Note|(hd0,0) numbering may change depending on your configuration. Reference: http://lists.mandriva.com/bugs/2009-08/msg03424.php.}}
+
 
+
Once you have installed GRUB, switch back to the main terminal with {{Keypress|Ctrl+Alt+F1}}.
+
 
+
Further information on paravirtualization with KVM can be found [http://www.linux-kvm.org/page/Boot_from_virtio_block_device here].
+
 
+
=== Preparing a Windows guest ===
+
 
+
Preparing a Windows guest for running with a virtio disk driver is a bit tricky.
+
 
+
In your KVM host (running Arch Linux), download the virtio disk driver from the [http://www.linux-kvm.org/page/WindowsGuestDrivers/Download_Drivers Fedora repository].
+
 
+
Now you need to create a new disk image, which fill force Windows to search for the driver. To do it, stop the virtual machine if its running and issue the following command:
+
$ qemu-img create -f qcow2 fake.img 1G
+
 
+
Run the original Windows guest (still in the IDE mode). Add the fake disk and a CD-ROM with the driver.
+
$ qemu-kvm -drive file=windows.img,if=ide -m 512 -drive file=fake.img,if=virtio -cdrom virtio-win-0.1-30.iso -vga std
+
 
+
If you have problems booting windows.img ISO image, or the virtio cd drivers not being detected, use this command.
+
$ qemu-kvm -drive file=fake.img,if=virtio -m 512 -boot d -drive file=windows.img,media=cdrom -drive file=virtio-win-0.1-30.iso,media=cdrom
+
 
+
Windows will detect the fake disk and try to find a driver for it. If it fails, go to Device Manager, locate the SCSI drive with an exclamation mark icon (should be open), click "Update driver" and select the virtual CD-ROM. Don't forget to mark the checkbox which says to search for directories recursively.
+
 
+
When the installation is successful, you can turn off the virtual machine and launch it again, now with the {{Ic|virtio}} driver.
+
 
+
$ qemu-kvm -drive file=windows.img,if=virtio -m 512 -vga std
+
 
+
{{Note|If you encounter the Blue Screen of Death, make sure you did not forget the {{ic|-m}} parameter, and that you do not boot with virtio instead of ide for the system drive before drivers are installed.}}
+
 
+
{{Note|The flag "boot&#61;on" was removed due to newer versions no need it anymore.}}
+
 
+
Preparing virtio networkdrivers is a bit easier, simply add the {{ic|-net}} argument as explained above.
+
$ qemu-kvm -drive file=windows.img,if=virtio -m 512 -vga std -net nic,model=virtio -cdrom virtio-win-cdrom virtio-win-0.1-30.iso
+
 
+
Then install the virtio drivers from the disk you downloaded; Go to the Device Manager, locate the network adapter with an exclamation mark icon (should be open), click "Update driver" and select the virtual CD-ROM. Don't forget to mark the checkbox which says to search for directories recursively.
+
 
+
=== Preparing a FreeBSD guest ===
+
 
+
Install the {{ic|emulators/virtio-kmod}} port if you are using FreeBSD 8.3 or later up until 10.0-CURRENT where they are included into the kernel. After installation, add the following to your {{ic|/boot/loader.conf}} file:
+
 
+
{{bc|<nowiki>
+
virtio_loader="YES"
+
virtio_pci_load="YES"
+
virtio_blk_load="YES"
+
if_vtnet_load="YES"
+
virtio_balloon_load="YES"
+
 
</nowiki>}}
 
</nowiki>}}
  
Then modify your {{ic|/etc/fstab}} by doing the following:
+
Verify that feature is activated:
 
+
{{hc|<nowiki>$ systool -m kvm_intel -v | grep nested</nowiki>|<nowiki>
{{bc|<nowiki>
+
    nested              = "Y"
sed -i/etc/fstab.bak "s/ad/vtbd/g" /etc/fstab
+
 
</nowiki>}}
 
</nowiki>}}
  
And verify that {{ic|/etc/fstab}} is consistent. If anything goes wrong, just boot into a rescue CD and copy {{ic|/etc/fstab.bak}} back to {{ic|/etc/fstab}}.
+
Run guest VM with following command:
 +
$ qemu-system-x86_64 -enable-kvm -cpu host
  
== Enabling KSM ==
+
Boot VM and check if vmx flag is present:
 
+
  $ grep vmx /proc/cpuinfo
Kernel Samepage Merging (KSM) is a feature of the Linux kernel introduced in the 2.6.32 kernel. KSM allows for an application to register with the kernel to have its pages merged with other processes that also register to have their pages merged. For KVM, the KSM mechanism allows for guest virtual machines to share pages with each other. In an environment where many of the guest operating systems are similar, this can result in significant memory savings.
+
 
+
There should be a {{ic|/sys/kernel/mm/ksm/}} directory containing several files. You can turn KSM on or off by echoing a {{ic|1}} or {{ic|0}} (respectively) to {{ic|/sys/kernel/mm/ksm/run}}:
+
# echo 1 > /sys/kernel/mm/ksm/run
+
Or set it up by creating the file {{ic|/etc/tmpfiles.d/ksm.conf}}:
+
w /sys/kernel/mm/ksm/run - - - - 1
+
If KSM is running, and there are pages to be merged (i.e. more than one similar VM is running), then {{ic|/sys/kernel/mm/ksm/pages_shared}} should be non-zero. From the kernel documentation in {{ic|Documentation/vm/ksm.txt}}:
+
The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
+
+
pages_shared    - how many shared unswappable kernel pages KSM is using
+
pages_sharing    - how many more sites are sharing them i.e. how much saved
+
pages_unshared  - how many pages unique but repeatedly checked for merging
+
pages_volatile  - how many pages changing too fast to be placed in a tree
+
full_scans      - how many times all mergeable areas have been scanned
+
+
A high ratio of pages_sharing to pages_shared indicates good sharing, but
+
a high ratio of pages_unshared to pages_sharing indicates wasted effort.
+
pages_volatile embraces several different kinds of activity, but a high
+
proportion there would also indicate poor use of madvise MADV_MERGEABLE.
+
 
+
An easy way to see how well KSM is performing is to simply print the contents of all the files in that directory.
+
 
+
{{hc|# grep . /sys/kernel/mm/ksm/*|
+
/sys/kernel/mm/ksm/full_scans:151
+
/sys/kernel/mm/ksm/max_kernel_pages:246793
+
/sys/kernel/mm/ksm/pages_shared:92112
+
/sys/kernel/mm/ksm/pages_sharing:131355
+
/sys/kernel/mm/ksm/pages_to_scan:100
+
/sys/kernel/mm/ksm/pages_unshared:123942
+
/sys/kernel/mm/ksm/pages_volatile:1182
+
/sys/kernel/mm/ksm/run:1
+
/sys/kernel/mm/ksm/sleep_millisecs:20
+
}}
+
 
+
== Enable HugePages ==
+
 
+
You may also want to enable hugepages to improve the performance of your virtual machine.
+
With an up to date Arch Linux and a running KVM you probably already have everything you need. Check if you have the directory {{ic|/dev/hugepages}}. If not create it.
+
Now we need the right permissions to use this directory. Check if the group {{ic|kvm}} exist and if you are member of this group. This should be the case if you already have a running virtual machine.
+
{{hc|$ getent group kvm|
+
kvm:x:78:USERNAMES
+
}}
+
 
+
Add to your {{ic|/etc/fstab}}:
+
hugetlbfs      /dev/hugepages  hugetlbfs      mode=1770,gid=78        0 0
+
Of course the gid must match that of the {{ic|kvm}} group. The mode of {{ic|1770}} allows anyone in the group to create files but not unlink or rename each other's files. Make sure {{ic|/dev/hugepages}} is mounted properly:
+
 
+
{{hc|# umount /dev/hugepages
+
# mount /dev/hugepages
+
$ mount <nowiki>|</nowiki> grep huge|
+
2=hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,mode=1770,gid=78)
+
}}
+
 
+
Now you can calculate how many hugepages you need. Check how large your hugepages are:
+
  $ cat /proc/meminfo | grep Hugepagesize
+
Normally that should be 2048 kB ≙ 2 MB. Let's say you want to run your virtual machine with 1024 MB. 1024 / 2 = 512. Add a few extra so we can round this up to 550. Now tell your machine how many hugepages you want:
+
# echo 550 > /proc/sys/vm/nr_hugepages
+
If you had enough free memory you should see:
+
{{hc|$ cat /proc/meminfo <nowiki>|</nowiki> grep HugePages_Total|
+
HugesPages_Total:  550
+
}}
+
If the number is smaller, close some applications or start your virtual machine with less memory (number_of_pages x 2):
+
$ kvm -m 1024 -mem-path /dev/hugepages [-hda yourimage.img] [-your_other_options]
+
Note the {{ic|-mem-path}} parameter. This will make use of the hugepages.
+
You can check now, while your virtual machine is running, how many pages are used:
+
{{hc|$ cat /proc/meminfo <nowiki>|</nowiki> grep HugePages|
+
HugePages_Total:    550
+
HugePages_Free:      48
+
HugePages_Rsvd:        6
+
HugePages_Surp:        0
+
}}
+
 
+
Now that everything seems to work you can enable hugepages by default if you like. Add to your {{ic|/etc/sysctl.conf}}:
+
vm.nr_hugepages = 550
+
 
+
See also: [http://wiki.debian.org/Hugepages Debian wiki] and [http://www.linux-kvm.com/content/get-performance-boost-backing-your-kvm-guest-hugetlbfs linux-kvm.com].
+
 
+
== Bridged Networking ==
+
 
+
=== Using netcfg ===
+
{{Out of date|Netcfg has been superseded by [[netctl]].}}
+
 
+
Bridged networking is used when you want your VM to be on the same network as your host machine. This will allow it to get a static or DHCP IP address on your network, and then you can access it using that IP address from anywhere on your LAN. The preferred method for setting up bridged networking for KVM is to use the {{Pkg|netcfg}} package. You will also need to install {{Pkg|bridge-utils}}.
+
 
+
For more information, see: [[Netcfg Tips#Configuring a bridge for use with virtual machines (VMs)]]
+
 
+
You can follow this page to configure the bridge: [[Libvirt#Bridged Networking]]
+
 
+
=== Additional notes ===
+
 
+
Other information can be found here: [[QEMU#Tap Networking with QEMU]] and [[QEMU#Networking with VDE2]].
+
 
+
If you are using {{Pkg|iptables}}, it is recommended for performance and security reasons to disable the firewall on the bridge:
+
# cat >> /etc/sysctl.conf <<EOF
+
net.bridge.bridge-nf-call-ip6tables = 0
+
net.bridge.bridge-nf-call-iptables = 0
+
net.bridge.bridge-nf-call-arptables = 0
+
EOF
+
# sysctl -p /etc/sysctl.conf
+
 
+
See the [http://wiki.libvirt.org/page/Networking#Creating_network_initscripts libvirt wiki] and [https://bugzilla.redhat.com/show_bug.cgi?id=512206 Fedora bug 512206]. If you get errors by sysctl during init (boot) about non-existing files, make the {{ic|bridge}} module load at boot. See [[Kernel_modules#Loading]].
+
 
+
Alternatively, you can configure {{Pkg|iptables}} to allow all traffic to be forwarded across the bridge by adding a rule like this:
+
-I FORWARD -m physdev --physdev-is-bridged -j ACCEPT
+
 
+
== Tips and tricks ==
+
 
+
{{Note|See [[QEMU#Tips and tricks]] and [[QEMU#Troubleshooting]] for more tricks.}}
+
  
 
=== Live snapshots ===
 
=== Live snapshots ===
Line 315: Line 135:
  
 
=== Poor Man's Networking ===
 
=== Poor Man's Networking ===
 +
 +
{{Merge|QEMU|This section is not KVM-specific, it's generally applicable to all QEMU VMs.}}
  
 
Setting up bridged networking can be a bit of a hassle sometimes. If the sole purpose of the VM is experimentation, one strategy to connect the host and the guests is to use SSH tunneling.
 
Setting up bridged networking can be a bit of a hassle sometimes. If the sole purpose of the VM is experimentation, one strategy to connect the host and the guests is to use SSH tunneling.
Line 342: Line 164:
 
This is a quite basic strategy to do networking with VMs. However, it is very robust and should be quite sufficient most of the time.
 
This is a quite basic strategy to do networking with VMs. However, it is very robust and should be quite sufficient most of the time.
  
=== Nested virtualization ===
+
{{Accuracy|Isn't this option enough? I think it should have the same effect: {{ic|-redir tcp:2222:10.0.2.15:22}} (it redirects port 2222 from host to 10.0.2.15:22, where 10.0.2.15 is guest's IP address.}}
  
{{Poor writing|The wrapper part is really terrible, and maybe unnecessary.}}
+
=== Enabling huge pages ===
  
Enable nested feature for kvm_intel:
+
{{Accuracy|With systemd, {{ic|hugetlbfs}} is mounted on {{ic|/dev/hugepages}} by default, but with mode 0755 and root's uid and gid.}}
  modprobe -r kvm_intel
+
{{Merge|QEMU|{{Pkg|qemu-kvm}} no longer exists as all of its features have been merged into {{Pkg|qemu}}. After the above issue is cleared, I suggest merging this section into [[QEMU]].}}
  modprobe kvm_intel nested=1
+
  
Verify that feature is activated: 
+
You may also want to enable hugepages to improve the performance of your virtual machine.
  # systool -m kvm_intel -v | grep nested
+
With an up to date Arch Linux and a running KVM you probably already have everything you need. Check if you have the directory {{ic|/dev/hugepages}}. If not create it.
    nested              = "Y"
+
Now we need the right permissions to use this directory. Check if the group {{ic|kvm}} exist and if you are member of this group. This should be the case if you already have a running virtual machine.
 +
{{hc|$ getent group kvm|
 +
kvm:x:78:USERNAMES
 +
}}
  
Create wrapper around qemu-kvm:
+
Add to your {{ic|/etc/fstab}}:
  # cat /usr/bin/qemu-kvm-nested
+
hugetlbfs      /dev/hugepages  hugetlbfs      mode=1770,gid=78        0 0
  #!/bin/bash
+
 
  /usr/bin/qemu-system-x86_64 -cpu host "$@"
+
Of course the gid must match that of the {{ic|kvm}} group. The mode of {{ic|1770}} allows anyone in the group to create files but not unlink or rename each other's files. Make sure {{ic|/dev/hugepages}} is mounted properly:
  # chmod a+x /usr/bin/qemu-kvm-nested
+
{{hc|# umount /dev/hugepages
  # ls -la /usr/bin/qemu-kvm
+
# mount /dev/hugepages
  lrwxrwxrwx 1 root root 18 29 oct.  15:38 /usr/bin/qemu-kvm -> qemu-system-x86_64
+
$ mount <nowiki>|</nowiki> grep huge|
  # ln -s /usr/bin/qemu-kvm-nested /usr/bin/qemu-kvm
+
2=hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,mode=1770,gid=78)
  # ls -la /usr/bin/qemu-kvm
+
}}
  lrwxrwxrwx 1 root root 24 12 nov13:09 /usr/bin/qemu-kvm -> /usr/bin/qemu-kvm-nested
+
 
 +
Now you can calculate how many hugepages you need. Check how large your hugepages are:
 +
  $ cat /proc/meminfo | grep Hugepagesize
 +
 
 +
Normally that should be 2048 kB ≙ 2 MB. Let's say you want to run your virtual machine with 1024 MB. 1024 / 2 = 512. Add a few extra so we can round this up to 550. Now tell your machine how many hugepages you want:
 +
# echo 550 > /proc/sys/vm/nr_hugepages
  
Boot VM and check if vmx flag is present:
+
If you had enough free memory you should see:
  grep vmx /proc/cpuinfo
+
{{hc|$ cat /proc/meminfo <nowiki>|</nowiki> grep HugePages_Total|
 +
HugesPages_Total:  550
 +
}}
 +
 
 +
If the number is smaller, close some applications or start your virtual machine with less memory (number_of_pages x 2):
 +
$ qemu-system-x86_64 -enable-kvm -m 1024 -mem-path /dev/hugepages -hda <disk_image> [...]
 +
 
 +
Note the {{ic|-mem-path}} parameter. This will make use of the hugepages.
 +
 
 +
Now you can check, while your virtual machine is running, how many pages are used:
 +
{{hc|$ cat /proc/meminfo <nowiki>|</nowiki> grep HugePages|
 +
HugePages_Total:    550
 +
HugePages_Free:      48
 +
HugePages_Rsvd:        6
 +
HugePages_Surp:        0
 +
}}
 +
 
 +
Now that everything seems to work you can enable hugepages by default if you like. Add to your {{ic|/etc/sysctl.conf}}:
 +
vm.nr_hugepages = 550
 +
 
 +
See also:
 +
* https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt
 +
* http://wiki.debian.org/Hugepages
 +
* http://www.linux-kvm.com/content/get-performance-boost-backing-your-kvm-guest-hugetlbfs
 +
 
 +
== See also ==
 +
* [http://www.linux-kvm.org/page/HOWTO KVM Howto]
 +
* [http://www.linux-kvm.org/page/FAQ#General_KVM_information KVM FAQ]

Revision as of 16:40, 16 September 2013

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary end KVM, Kernel-based Virtual Machine, is a hypervisor built into the Linux kernel. It is similar to Xen in purpose but much simpler to get running. Unlike native QEMU, which uses emulation, KVM is a special operating mode of QEMU that uses CPU extensions (HVM) for virtualization via a kernel module. KVM originally supported x86 and x86_64 architectures and has been ported to S/390, PowerPC, IA-64, and ARM (since Linux 3.9).

Using KVM, one can run multiple virtual machines running unmodified GNU/Linux, Windows, or any other operating system. (See Guest Support Status for more information.) Each virtual machine has private virtualized hardware: a network card, disk, graphics card, etc.

Differences among KVM, Xen, VMware, and QEMU can be found at the KVM FAQ.

Checking support for KVM

Hardware support

KVM requires that the virtual machine host's processor has virtualization support (named VT-x for Intel processors and AMD-V for AMD processors). You can check whether your processor supports hardware virtualization with the following command:

$ lscpu

Your processor supports virtualization only if there is a line telling you so.

You can also run:

$ grep -E "(vmx|svm)" --color=always /proc/cpuinfo

If nothing is displayed after running that command, then your processor does not support hardware virtualization, and you will not be able to use KVM.

Kernel support

You can check if necessary modules (kvm and one of kvm_amd, kvm_intel) are available in your kernel with the following command (assuming your kernel is built with CONFIG_IKCONFIG_PROC):

$ zgrep KVM /proc/config.gz
Note: Arch Linux kernels provide the appropriate kernel modules to support KVM.

Loading kernel modules

You need to load kvm module and one of kvm_amd and kvm_intel depending on the manufacturer of the VM host's CPU. See Kernel modules#Loading and Kernel modules#Manual module handling for information about loading kernel modules.

If modprobing kvm_intel or kvm_amd fails but modprobing kvm succeeds, (and lscpu claims that hardware acceleration is supported), check your BIOS settings. Some vendors (especially laptop vendors) disable these processor extensions by default. To determine whether there's no hardware support or there is but the extensions are disabled in BIOS, the output from dmesg after having failed to modprobe will tell.

Note: Newer versions of udev should load these modules automatically, so manual intervention is not required.

How to use KVM

See the main article: QEMU.

Tips and tricks

Note: See QEMU#Tips and tricks and QEMU#Troubleshooting for general tips and tricks.

Nested virtualization

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Is it possible also with kvm_amd? (Discuss in Talk:KVM#)

On host, enable nested feature for kvm_intel:

# modprobe -r kvm_intel
# modprobe kvm_intel nested=1

To make it permanent (see Kernel modules#Setting module options):

/etc/modprobe.d/modprobe.conf
options kvm_intel nested=1

Verify that feature is activated:

$ systool -m kvm_intel -v | grep nested
    nested              = "Y"

Run guest VM with following command:

$ qemu-system-x86_64 -enable-kvm -cpu host

Boot VM and check if vmx flag is present:

$ grep vmx /proc/cpuinfo

Live snapshots

Merge-arrows-2.pngThis article or section is a candidate for merging with libvirt.Merge-arrows-2.png

Notes: virsh is part of libvirt (Discuss in Talk:KVM#)

A feature called external snapshotting allows one to take a live snapshot of a virtual machine without turning it off. Currently it only works with qcow2 and raw file based images.

Once a snapshot is created, KVM attaches that new snapshotted image to virtual machine that is used as its new block device, storing any new data directly to it while the original disk image is taken offline which you can easily copy or backup. After that you can merge the snapshotted image to the original image, again without shutting down your virtual machine.

Here's how it works.

Current running vm

# virsh list --all
Id    Name                           State
----------------------------------------------------
3     archey                            running

List all its current images

# virsh domblklist archey 
Target     Source
------------------------------------------------
vda        /vms/archey.img

Notice the image file properties

# qemu-img info /vms/archey.img
image: /vms/archey.img
file format: qcow2
virtual size: 50G (53687091200 bytes)
disk size: 2.1G
cluster_size: 65536

Create a disk-only snapshot. The switch --atomic makes sure that the VM is not modified if snapshot creation fails.

# virsh snapshot-create-as archey snapshot1 --disk-only --atomic

List if you want to see the snapshots

# virsh snapshot-list archey
Name                 Creation Time             State
------------------------------------------------------------
snapshot1           2012-10-21 17:12:57 -0700 disk-snapshot

Notice the new snapshot image created by virsh and its image properties. It weighs just a few MiBs and is linked to its original "backing image/chain".

# qemu-img info /vms/archey.snapshot1
image: /vms/archey.snapshot1
file format: qcow2
virtual size: 50G (53687091200 bytes)
disk size: 18M
cluster_size: 65536
backing file: /vms/archey.img

At this point, you can go ahead and copy the original image with cp -sparse=true or rsync -S. Then you can merge the original image back into the snapshot.

# virsh blockpull --domain archey --path /vms/archey.snapshot1

Now that you have pulled the blocks out of original image, the file /vms/archey.snapshot1 becomes the new disk image. Check its disk size to see what it means. After that is done, the original image /vms/archey.img and the snapshot metadata can be deleted safely. The virsh blockcommit would work opposite to blockpull but it seems to be currently under development in qemu-kvm 1.3 (including snapshot-revert feature), scheduled to be released sometime next year.

This new feature of KVM will certainly come handy to the people who like to take frequent live backups without risking corruption of the file system.

Poor Man's Networking

Merge-arrows-2.pngThis article or section is a candidate for merging with QEMU.Merge-arrows-2.png

Notes: This section is not KVM-specific, it's generally applicable to all QEMU VMs. (Discuss in Talk:KVM#)

Setting up bridged networking can be a bit of a hassle sometimes. If the sole purpose of the VM is experimentation, one strategy to connect the host and the guests is to use SSH tunneling.

The basic steps are as follows:

  • Setup an SSH server in the host OS
  • (optional) Create a designated user used for the tunneling (e.g. tunneluser)
  • Install SSH in the VM
  • Setup authentication

See: SSH for the setup of SSH, especially SSH#Forwarding_Other_Ports.

When using the default user network stack, the host is reachable at address 10.0.2.2.

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements.Tango-edit-clear.png

Reason: Usage of /etc/rc.local is discouraged. This should be a proper systemd service file. (Discuss in Talk:KVM#)

If everything works and you can SSH into the host, simply add something like the following to your /etc/rc.local

# Local SSH Server
echo "Starting SSH tunnel"
sudo -u vmuser ssh tunneluser@10.0.2.2 -N -R 2213:127.0.0.1:22 -f
# Random remote port (e.g. from another VM)
echo "Starting random tunnel"
sudo -u vmuser ssh tunneluser@10.0.2.2 -N -L 2345:127.0.0.1:2345 -f

In this example a tunnel is created to the SSH server of the VM and an arbitrary port of the host is pulled into the VM.

This is a quite basic strategy to do networking with VMs. However, it is very robust and should be quite sufficient most of the time.

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Isn't this option enough? I think it should have the same effect: -redir tcp:2222:10.0.2.15:22 (it redirects port 2222 from host to 10.0.2.15:22, where 10.0.2.15 is guest's IP address. (Discuss in Talk:KVM#)

Enabling huge pages

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: With systemd, hugetlbfs is mounted on /dev/hugepages by default, but with mode 0755 and root's uid and gid. (Discuss in Talk:KVM#)

Merge-arrows-2.pngThis article or section is a candidate for merging with QEMU.Merge-arrows-2.png

Notes: qemu-kvm no longer exists as all of its features have been merged into qemu. After the above issue is cleared, I suggest merging this section into QEMU. (Discuss in Talk:KVM#)

You may also want to enable hugepages to improve the performance of your virtual machine. With an up to date Arch Linux and a running KVM you probably already have everything you need. Check if you have the directory /dev/hugepages. If not create it. Now we need the right permissions to use this directory. Check if the group kvm exist and if you are member of this group. This should be the case if you already have a running virtual machine.

$ getent group kvm
kvm:x:78:USERNAMES

Add to your /etc/fstab:

hugetlbfs       /dev/hugepages  hugetlbfs       mode=1770,gid=78        0 0

Of course the gid must match that of the kvm group. The mode of 1770 allows anyone in the group to create files but not unlink or rename each other's files. Make sure /dev/hugepages is mounted properly:

# umount /dev/hugepages
# mount /dev/hugepages
$ mount | grep huge
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,mode=1770,gid=78)

Now you can calculate how many hugepages you need. Check how large your hugepages are:

$ cat /proc/meminfo | grep Hugepagesize

Normally that should be 2048 kB ≙ 2 MB. Let's say you want to run your virtual machine with 1024 MB. 1024 / 2 = 512. Add a few extra so we can round this up to 550. Now tell your machine how many hugepages you want:

# echo 550 > /proc/sys/vm/nr_hugepages

If you had enough free memory you should see:

$ cat /proc/meminfo | grep HugePages_Total
HugesPages_Total:  550

If the number is smaller, close some applications or start your virtual machine with less memory (number_of_pages x 2):

$ qemu-system-x86_64 -enable-kvm -m 1024 -mem-path /dev/hugepages -hda <disk_image> [...]

Note the -mem-path parameter. This will make use of the hugepages.

Now you can check, while your virtual machine is running, how many pages are used:

$ cat /proc/meminfo | grep HugePages
HugePages_Total:     550
HugePages_Free:       48
HugePages_Rsvd:        6
HugePages_Surp:        0

Now that everything seems to work you can enable hugepages by default if you like. Add to your /etc/sysctl.conf:

vm.nr_hugepages = 550

See also:

See also