Linux Virtualization - Jul 2015 - [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

On 28/07/2015 15:11, Jan Kiszka wrote:
>>> >>
>>> >> This doesn't matter much, since the only guests that
implement an IOMMU
>>> >> in QEMU are (afaik) PPC and x86, and x86 does not yet
promise any kind
>>> >> of stability.
>> > 
>> > Hmm I think Jan (cc) said it was already used out there.
> Yes, no known issues with vt-d emulation for almost a year now. Error
> reporting could be improved, and interrupt remapping is still missing,
> but those are minor issues in this context.
On the other hand interrupt remapping is absolutely necessary for
production use, hence my point that x86 does not promise API stability.

("Any kind of stability" actually didn't include crashes; those
are not
expected :))

The Google patches for userspace PIC and IOAPIC are proceeding well, so
hopefully we can have interrupt remapping soon.

Paolo

On 2015-07-28 18:36, Paolo Bonzini wrote:
> On 28/07/2015 15:11, Jan Kiszka wrote:
>>>>>>
>>>>>> This doesn't matter much, since the only guests
that implement an IOMMU
>>>>>> in QEMU are (afaik) PPC and x86, and x86 does not yet
promise any kind
>>>>>> of stability.
>>>>
>>>> Hmm I think Jan (cc) said it was already used out there.
>> Yes, no known issues with vt-d emulation for almost a year now. Error
>> reporting could be improved, and interrupt remapping is still missing,
>> but those are minor issues in this context.
> 
> On the other hand interrupt remapping is absolutely necessary for
> production use, hence my point that x86 does not promise API stability.
Well, we currently implement the features that the Q35 used to expose.
Adding interrupt remapping will require a new chipset and/or a hack
switch to ignore compatibility.
> 
> ("Any kind of stability" actually didn't include crashes;
those are not
> expected :))
> 
> The Google patches for userspace PIC and IOAPIC are proceeding well, so
> hopefully we can have interrupt remapping soon.
If the day had 48 hours... I'd love to look into this, first adding QEMU
support for the new irqchip architecture.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

On 28/07/2015 18:42, Jan Kiszka wrote:
> > On the other hand interrupt remapping is absolutely necessary for
> > production use, hence my point that x86 does not promise API
stability.
> 
> Well, we currently implement the features that the Q35 used to expose.
> Adding interrupt remapping will require a new chipset and/or a hack
> switch to ignore compatibility.
Isn't the VT-d register space separate from other Q35 features and
backwards-compatible?  You could even add it to PIIX in theory just by
adding a DMAR.

It's not like for example SMRAM, where the registers are in the
northbridge configuration space and move around in every chipset generation.
> > ("Any kind of stability" actually didn't include
crashes; those are not
> > expected :))
> > 
> > The Google patches for userspace PIC and IOAPIC are proceeding well,
so
> > hopefully we can have interrupt remapping soon.
> 
> If the day had 48 hours... I'd love to look into this, first adding
QEMU
> support for the new irqchip architecture.
I hope I can squeeze in some time for that...  Google also had an intern
that was looking at it.

Paolo