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qm: add section about machine types

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expanding from the two currently existing sentences. In the first one, a
typo VMs -> VM's is fixed. In the second one, "one wants to" is changed
to "you want to", because the sentence already starts with "You can" and
it's active voice.

Adds information about the machine version, rationale behind the
defaults for it and what to do for deprecated versions.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
This commit is contained in:
Fiona Ebner 2023-11-10 14:24:45 +01:00 committed by Thomas Lamprecht
parent 9e16e514c3
commit cecc8064dc
1 changed files with 37 additions and 5 deletions
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@ -139,12 +139,44 @@ snapshots) more intelligently.
{pve} allows to boot VMs with different firmware and machine types, namely
xref:qm_bios_and_uefi[SeaBIOS and OVMF]. In most cases you want to switch from
the default SeaBIOS to OVMF only if you plan to use
xref:qm_pci_passthrough[PCIe passthrough]. A VMs 'Machine Type' defines the
hardware layout of the VM's virtual motherboard. You can choose between the
default https://en.wikipedia.org/wiki/Intel_440FX[Intel 440FX] or the
xref:qm_pci_passthrough[PCIe passthrough].
Machine Type
^^^^^^^^^^^^
A VM's 'Machine Type' defines the hardware layout of the VM's virtual
motherboard. You can choose between the default
https://en.wikipedia.org/wiki/Intel_440FX[Intel 440FX] or the
https://ark.intel.com/content/www/us/en/ark/products/31918/intel-82q35-graphics-and-memory-controller.html[Q35]
chipset, which also provides a virtual PCIe bus, and thus may be desired if
one wants to pass through PCIe hardware.
chipset, which also provides a virtual PCIe bus, and thus may be
desired if you want to pass through PCIe hardware.
Each machine type is versioned in QEMU and a given QEMU binary supports many
machine versions. New versions might bring support for new features, fixes or
general improvements. However, they also change properties of the virtual
hardware. To avoid sudden changes from the guest's perspective and ensure
compatibility of the VM state, live-migration and snapshots with RAM will keep
using the same machine version in the new QEMU instance.
For Windows guests, the machine version is pinned during creation, because
Windows is sensitive to changes in the virtual hardware - even between cold
boots. For example, the enumeration of network devices might be different with
different machine versions. Other OSes like Linux can usually deal with such
changes just fine. For those, the 'Latest' machine version is used by default.
This means that after a fresh start, the newest machine version supported by the
QEMU binary is used (e.g. the newest machine version QEMU 8.1 supports is
version 8.1 for each machine type).
Very old machine versions might become deprecated in QEMU. For example, this is
the case for versions 1.4 to 1.7 for the i440fx machine type. It is expected
that support for these machine versions will be dropped at some point. If you
see a deprecation warning, you should change the machine version to a newer one.
Be sure to have a working backup first and be prepared for changes to how the
guest sees hardware. In some scenarios, re-installing certain drivers might be
required. You should also check for snapshots with RAM that were taken with
these machine versions (i.e. the `runningmachine` configuration entry).
Unfortunately, there is no way to change the machine version of a snapshot, so
you'd need to load the snapshot to salvage any data from it.
[[qm_hard_disk]]
Hard Disk