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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: Thu, 14 Apr 2016 16:28:02 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Apr 13, 2016 at 10:42:26PM -0700, Jan Kiszka wrote:
> On 2016-04-13 19:46, Peter Xu wrote:
> > On Wed, Apr 13, 2016 at 07:44:54AM -0700, Jan Kiszka wrote:
> > [...]
> >>> 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
> >>> generated."
> >>>
> >>> So... In real IOMMU hardwares, IOAPIC interrupt will finally be
> >>> translated to MSI as well? am I understanding it correctly?
> >>
> >> It will be translated into something that the IR unit can receive -
> >> whatever that is technically. Logically, there is no difference to MSIs
> >> received from PCI devices.
> > 
> > Ok. I see there are still differences between IOAPIC and MSI, e.g.,
> > for IOAPIC entries, it has "Interrupt Input Pin Polarity", which is
> > bit 13 of the entry, to show whether 1 or 0 is taken as assertion
> > for level-triggered interrupts. While in MSI, I assume assertion
> > will be 1 always. Can I take it as "obsolete" and we will never use
> > it?
> I can't check details right now, but I would recommend studying in the
> specs if any of the *receiver* (APIC and IOMMU) can make sense of that
> bit at all, and how. Also study (commit logs) if there is a reason why
> the bit is unused.

Thanks for the advices. Will add this in my todo list. I am guessing
that, all devices emulated by QEMU are using 1 as assertions..

It's defined as IOAPIC_LVT_POLARITY in QEMU. As long as I am sure
that current QEMU does not use it, I think I can move on to v3 using
MSI path, at least to make sure I'm not making things worse, or
bringing any functionality loss.

> > 
> > If to take IOAPIC entries as MSI messages, all these extra bits will
> > be dropped (it's dropped in ioapic_service() already, but not sure
> > whether we will pick them up in the future).
> What other bits?

Besides polarity bit, there is another one IOAPIC_LVT_DELIV_STATUS,
which is not used too. But after more careful look, I see this bit
is not too relevant if we are talking about translating IOAPIC
entries into MSI.


-- peterx

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