Re: [Qemu-devel] [PATCH v2 12/13] intel_iommu: ioapic: IR support for emulated IOAPIC

[Peter Xu <peterx@redhat.com>]

On Tue, Apr 12, 2016 at 08:39:02AM -0700, Jan Kiszka wrote:
> On 2016-04-12 02:02, Peter Xu wrote:

[...]

> > Yes, I should consider other x86 platforms like AMD. Thanks to point
> > out. It seems that there are many places in the patchset that lacks
> > thorough consideration about this. Will try to fix them in next
> > version.
> > 
> > Regarding to the above MSI solution: I'd say it is a good way to
> > hide everything else behind.  However, since we introduced one extra
> > layer (MSI) which actually does not exist, not sure there would be
> > problem too.  Also, I feel it a little bit hacky if we "create" one
> > MSI out of the air...  For example, if someone tries to capture MSIs
> > from QEMU inside in the APIC memory writes, he will see something he
> > cannot explain if he never knows this hack's there.  Considering the
> > above, I would prefer hooks, or better to provide a callback (a
> > function pointer that others like AMD can override) to do the
> > translation.  How do you think?
> 
> The HPET does send MSIs, and I'm not sure how much different the
> IOAPIC's message actually is. In any case, modelling it as MSI is
> neither adding incorrectness nor making the code more complex (in fact,
> the contrary is true!). Last but not least, it would be trivial to
> filter out non-PCI MSI sources if we wanted to trace only PCI - because
> we need to identify the origin anyway for remapping purposes. So,
> explicit hooking looks like the wrong way to me.

I am just not sure about the difference between IOAPIC's messages
and MSI ones. For now, they seems very alike. However, I am not sure
whether it would be not alike in the future. E.g., if one day, we
extend APIC bus to support more than 255 CPUs (could it? I do not
know for sure), here if we are with this "MSI layer", we would not
be able to do that, since MSI only support 8 bits for destination ID
field. That's my only worry now. If you (or Radim? or anyone more
experienced on this than me) can confirm that this would never be a
problem, I'd be glad to take the MSI way.

Thanks.

-- peterx