Re: [Qemu-devel] vIOMMU Posted-interrupt implementation - atomic operation?

[Jintack Lim]

On Wed, Jun 6, 2018 at 2:56 AM, Tian, Kevin <address@hidden> wrote:
>> From: Jintack Lim [mailto:address@hidden
>> Sent: Tuesday, June 5, 2018 8:57 PM
>>
>> Thanks, Kevin.
>>
>> On Tue, Jun 5, 2018 at 2:54 AM, Tian, Kevin <address@hidden> wrote:
>> >> From: Jintack Lim
>> >> Sent: Friday, June 1, 2018 11:47 AM
>> >>
>> >> Hi,
>> >>
>> >> I'm implementing Posted-interrupt functionality in vIOMMU. According
>> >> to Vt-d spec 5.2.3, IOMMU performs a coherent atomic read-modify-
>> write
>> >> operation of the posted-interrupt descriptor. I wonder how can we
>> >> achieve this considering the guest can modify the same
>> >> posted-interrupt descriptor anytime. Is there any existing mechanism
>> >> that I can use in QEMU?
>> >>
>> >
>> > I don't think it's possible to emulate such operation in software, unless
>> > you want to change guest to be cooperative. Actually it is not necessary.
>> > VT-d does so due to some hardware implementation consideration.
>>
>> Would you mind expanding this? I'm curious what it would be. Is it
>> because IOMMU can't do something like cmpxchg instructions?
>
> I don't have further information. Above is what I was told by hardware
> team.

Ah, I see. Thanks!

>
>>
>> > Since you are emulating on CPU, could just follow how CPU posted
>> > interrupt is conducted. If you look at SDM (29.6 Posted-Interrupt
>> > Processing):
>> >
>> >         "There is a requirement, however, that such modifications be
>> > done using locked read-modify-write instructions."
>> >
>> > [instructions] means you can do update multiple times when posting an
>> > interrupt, as long as each update is atomic.
>>
>> Ah, that's a good point. So the unit of atomic operation doesn't need
>> to be the whole PI descriptor, but it can be any subset (e.g. just one
>> bit) of the descriptor? By looking at Linux kernel code, that seems to
>> be the case.
>>
>
> Exactly. :-)

Cool. Thanks for the confirmation.

Thanks,
Jintack

>
> Thanks
> Kevin