Difference between revisions of "KVM"
m (→Poor Man's Networking: merge suggestion; added note to perhaps improve/expand the section) |
(→How to use KVM: remove note about qemu-kvm fork being merged into upstream QEMU) |
||
(26 intermediate revisions by 4 users not shown) | |||
Line 3: | Line 3: | ||
[[it:KVM]] | [[it:KVM]] | ||
[[zh-CN:KVM]] | [[zh-CN:KVM]] | ||
− | {{ | + | {{Article summary start}} |
− | {{ | + | {{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 | + | {{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]) | + | 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]. | ||
− | |||
− | == | + | == 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: | 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 | ||
− | + | 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. |
− | == | + | === Kernel support === |
− | + | 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}}): | |
− | + | $ zgrep KVM /proc/config.gz | |
− | |||
− | {{ | ||
− | + | {{Note|Arch Linux kernels provide the appropriate [[Kernel_modules|kernel modules]] to support KVM.}} | |
− | + | === Loading kernel modules === | |
− | |||
− | + | 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. | |
− | |||
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. | ||
− | + | {{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]]. | ||
− | + | == Tips and tricks == | |
− | + | {{Note|See [[QEMU#Tips and tricks]] and [[QEMU#Troubleshooting]] for general tips and tricks.}} | |
− | + | === Nested virtualization === | |
− | |||
− | + | {{Expansion|Is it possible also with {{ic|kvm_amd}}?}} | |
− | |||
− | |||
− | |||
− | |||
− | }} | ||
− | + | On host, enable nested feature for {{ic|kvm_intel}}: | |
− | + | # modprobe -r kvm_intel | |
+ | # modprobe kvm_intel nested=1 | ||
− | + | To make it permanent (see [[Kernel modules#Setting module options]]): | |
− | {{hc| | + | {{hc|/etc/modprobe.d/modprobe.conf|<nowiki> |
− | + | options kvm_intel nested=1 | |
− | + | </nowiki>}} | |
− | |||
− | }} | ||
− | + | Verify that feature is activated: | |
− | + | {{hc|<nowiki>$ systool -m kvm_intel -v | grep nested</nowiki>|<nowiki> | |
+ | nested = "Y" | ||
+ | </nowiki>}} | ||
− | + | 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 === | === Live snapshots === | ||
Line 225: | 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. | ||
− | {{ | + | {{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.}} |
+ | |||
+ | === Enabling huge pages === | ||
+ | |||
+ | {{Accuracy|With systemd, {{ic|hugetlbfs}} is mounted on {{ic|/dev/hugepages}} by default, but with mode 0755 and root's uid and gid.}} | ||
+ | {{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]].}} | ||
+ | |||
+ | 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): | |
+ | $ 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
zh-CN:KVM 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.
Contents
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
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.
How to use KVM
See the main article: QEMU.
Tips and tricks
Nested virtualization
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
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
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.
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.
Enabling huge pages
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:
- 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