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

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH v2 12/13] intel_iommu: ioapic: IR support for emulated IOAPIC
Date: Wed, 13 Apr 2016 18:06:43 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Tue, Apr 12, 2016 at 08:39:21PM -0700, Jan Kiszka wrote:
> On 2016-04-12 20:33, Peter Xu wrote:
> > 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.
> That's one of the reason why we need IR: >255 is only possible this way,
> because it requires x2APIC and that requires IR (see Intel spec). So,
> IOAPIC messages will then always travel via VT-d. No need to worry at all.

One thing I am curious about: I see that in vtd spec

"RTE bits 10:8 is programmed to 000b (Fixed) to force the SHV
(SubHandle Valid) field as Clear in the interrupt address

So... In real IOMMU hardwares, IOAPIC interrupt will finally be
translated to MSI as well? am I understanding it correctly?

Btw, if to use the way you suggested, the patch content would
possibly be very alike the one you and Rita has posted, which is:


I will only pick up those lines I needed in supporting IOAPIC. If
so, do you mind I add your s-o-b as well above mine (maybe add
Rita's too)?


-- peterx

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