msitranslate: question about qemu-upstream and pv support

msitranslate: question about qemu-upstream and pv support

[Andrew Sorensen]

I noticed that the patches for msitranslate do not seem to have made it
into qemu-upstream. The logic still exists in
xen/drivers/passthrough/io.c, though documentation suggests MSI-INTx
translation is only implemented for HVM guests.

Based on the disabling of MSI-INTx by default, it seems safe to assume
that MSI translation is only supported under qemu-xen-traditional, and
only for HVM guests. Is this correct?

I don't see any mention of this on this wiki page:
http://wiki.xen.org/wiki/QEMU_Upstream
#Missing_feature_from_the_good_old_qemu-dm

Re: msitranslate: question about qemu-upstream and pv support

[Konrad Rzeszutek Wilk]

On Thu, Jul 24, 2014 at 03:16:24PM -0700, Andrew Sorensen wrote:
> I noticed that the patches for msitranslate do not seem to have made it
> into qemu-upstream. The logic still exists in
> xen/drivers/passthrough/io.c, though documentation suggests MSI-INTx
> translation is only implemented for HVM guests.
> 
> Based on the disabling of MSI-INTx by default, it seems safe to assume
> that MSI translation is only supported under qemu-xen-traditional, and
> only for HVM guests. Is this correct?

My recollection is that it is an optimization. That is - if we do PCI
passthrough - and the device is doing legacy interrpupts - we want
to pass that (as it means updating QEMU) and instead doing an MSI - so
that QEMU is not scheduled in.

But most network cards don't use legacy interrupts. Perhaps the
video cards use it - but for example the i915 is doing MSI and
I would think that radeon and nouvau are moving that way too?

> 
> I don't see any mention of this on this wiki page:
> http://wiki.xen.org/wiki/QEMU_Upstream
> #Missing_feature_from_the_good_old_qemu-dm

That looks to be an omission. Do you have an account on the
Wiki to update it? Are you Ok updating it? Thank you!
> 
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