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

Stefan Reiter s.reiter at proxmox.com
Thu Oct 21 11:56:04 CEST 2021

On 10/21/21 11:47 AM, Thomas Lamprecht wrote:
> 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

I did read it, and I agree it has some improvements, was just
wondering if it was worth our effort here (never encountered any of
the described bugs or saw a user that encountered them anywhere).

But I don't think this series is as bad as you make it out to be
either ;)

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