[pve-devel] [PATCH 0/4] add meta info and bandaid for QEMU 6.1 and unpinned q35 machine backward compat

[Thomas Lamprecht]

On 21.10.21 11:34, Stefan Reiter wrote:
> On 10/21/21 10:36 AM, Thomas Lamprecht wrote:
>> First add a new meta property that is currently exclusively set on new
>> VM creation and then read-only, initially add the creation time as UNIX
>> epoch and the QEMU version that was installed during installation
>> (thought about using the one on first start but that actually does not
>> gives any more guarantee, so just go for simple).
>>
>> Use that information to band aid around a change regarding hotplug in
>> QEMU 6.1 that can affected older VMs on fresh start (migration and
>> rollback is covered by force-machine mechanisms as always already).
>>
>> I'm not 100% convinced of the whole thing, albeit I see some merit in
>> the meta property even if we do not go with the last patch, anyhow, I
>> proposed this off-list to Dominik (and those thing is partly his idea
>> too), Wolfgang, Fabian and Stefan and none of them rejected the idea nor
>> communicated a better/more preferred alternative, so I went for it
>> (still not steaming from enthusiasm though)
> 
> So we're doing all of this to avoid issues with older VMs that expect
> "acpi-pci-hotplug-with-bridge-support=off" on Q35 (previously default),
> but we still want to set it for new VMs that are created with QEMU 6.1
> and never booted with anything older.
> 
> But taking a step back, do we actually want the new ACPI hotplug in
> general? If we choose to simply leave it be, we could just always add
> "acpi-pci-hotplug-with-bridge-support=off" to Q35 on QEMU > 6.1.
> Since it's a global property, I think we wouldn't even need to check
> machine-type/forcemachine at all, since we'd only make the default
> explicit with older ones.

Check the commit I linked in patch 4/4, the change has some value.

https://gitlab.com/qemu-project/qemu/-/commit/17858a1695