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

[Jan Kiszka]

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 5.1.5.1:
>>>
>>> "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.

> 
> 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?

Jan