Re: [Qemu-devel] [PATCH v2 7/7] intel-iommu: keep buggy EIM enabled in 2.7 machine type

[Radim Krčmář]

2016-09-29 15:19+0200, Paolo Bonzini:
> On 29/09/2016 13:23, Radim Krčmář wrote:
>> QEMU 2.7 allowed EIM even in configurations that were forbidden in the
>> last patch because they were not working, like old KVM or userspace
>> APIC.  In order to keep backward compatibility, we again allow guests to
>> misbehave in non-obvious ways, and make it the default.
>> 
>> Signed-off-by: Radim Krčmář <address@hidden>
> 
> Ugh, I misremembered that VTD_ECAP_EIM was not set in 2.7. :(  Perhaps
> it's better to drop this patch...

I think that adding this backward compatibility hack into code that is
supposed to be developed is not a good idea.


2016-09-29 12:01-0300, Eduardo Habkost:
> If you break compatibility and fix it in separate patches, you
> break bisectability (even for people that are bisecting bugs
> unrelated to EIM).

I'd keep it as a separate patch and let maintainers decide whether they
want to squish or drop it.

> (But I still don't understand if patch 6/7 really breaks
> anything, or not.)

Nothing useful.
It "breaks" three cases:

1) If user configured

     -machine kernel_irqchip=off -device intel_iommu,intremap=on

   QEMU 2.7 pc-q35-2.7 enabled (broken) EIM, but 2.8 wouldn't, leading
   to a different machine.

   (The same with new KVM and split irqchip.)

2) If user had old KVM and configured

     -machine kernel_irqchip=split -device intel_iommu,intremap=on

   QEMU 2.7 pc-q35-2.7 enabled (broken) EIM, but after offline migration
   to 2.8, QEMU would refuse to start.

3) If user started a pc-q35-2.7 with QEMU 2.8 on a new KVM, then they
   could use cluster x2APIC without a problem, but the guest wouldn't
   work after offline migration to QEMU 2.7 (I'm not sure if this case
   is supported).

Luckily, the intel-iommu device doesn't support live migration. :)