Re: [PATCH v1 1/1] hw/pci: Disable PCI_ERR_UNCOR_MASK register for machine type < 8.0

[Juan Quintela <quintela@redhat.com>]

Michael Tokarev <mjt@tls.msk.ru> wrote:
> 11.05.2023 11:40, Juan Quintela wrote:
>> Fiona Ebner <f.ebner@proxmox.com> wrote:
> ...
>>> Closes: https://gitlab.com/qemu-project/qemu/-/issues/1576
>>>
>>> AFAICT, this breaks (forward) migration from 8.0 to 8.0 + this patch
>>> when using machine type <= 7.2. That is because after this patch, when
>>> using machine type <= 7.2, the wmask for the register is not set and
>>> when 8.0 sends a nonzero value for the register, the error condition in
>>> get_pci_config_device() will trigger again.
>> I think that works correctly.
>> See https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg02733.html
>> What we have (before this patch) (using abbrevs as in the doc
>> before)
>> Current state:
>> (1) qemu-8.0 -M pc-8.0 -> qemu-8.0 -M pc-8.0 works
>>      not affected by the patch
>> (2) qemu-7.2 -M pc-7.2 -> qemu-8.0 -M pc-8.0 works
>>      works well because 7.2 don't change that field
>> (3) qemu-8.0 -M pc-7.2 -> qemu-7.2 -M pc-7.2 fails
>> With the patch we fixed 3, so once it is in stable, 1 and 2 continue
>> as
>> usual and for (3) we will have:
>> (3) qemu-8.0.1 -M pc-7.2 -> qemu-7.2 -M pc-7.2 works
>> If what you mean is that:
>> (3) qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 works
>> Will fail, that is true, but I can think a "sane" way to fix this.

Hi

> That's a great summary indeed.

Thanks.

>>> Is it necessary to also handle that? Maybe by special casing the error
>>> condition in get_pci_config_device() to be prepared to accept such a
>>> stream from 8.0?
>> Well, we can do that, but it is to the pci maintainers to decide if
>> that
>> is "sane".
>
> So, can we go from here somewhere? I'd love this fix to be in 8.0.1,
> either with or without the (un)sane part of the (3) variant above which
> might fail.  Or else we'll have the same situation in 8.0.1 as we now
> have in 8.0.0 (the deadline is May-27).
>
> We did broke x.y.0 => x.y.1 migration before already like this, such as
> with 7.2.0=>7.2.1. I'm not saying it's a nice thing to do, just stating
> a fact. Yes, it is better to avoid such breakage, but.. meh..

See patch for documentation:

https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg03288.html

Basically, the best we can do is:
- get the patch posted.  Fixes everything except:
  (3) qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 works

And for that, we can document somewhere that we need to launch
qemu-8.0.1 as:

$ qemu-8.0.1 -M pc-7.2 -device blah,x-pci-err-unc-mask=on

And mark someone that this machine is tainted an can only be migrated to
qemu's >= qemu-8.0.1.  And that we should reboot it as the user
convenience. (reboot here means poweroff qemu and poweron it back
without x-pci-err-unc-mask=on).

Later, Juan.