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

[Jintack Lim]

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?

> 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.

Best,
Jintack

>
> Thanks
> Kevin
>