Re: [RFC PATCH 4/5] APIC/IOAPIC EOI callback

Avi Kivity
2010-07-12T09:14:25Z

On 07/12/2010 12:05 PM, Gleb Natapov wrote:
> On Mon, Jul 12, 2010 at 09:33:12AM +0300, Avi Kivity wrote:
>
>> On 07/11/2010 09:30 PM, Avi Kivity wrote:
>>
>>>> Registering an eventfd for the eoi seems like a reasonable alternative.
>>>>
>>> I'm worried about that racing (with what?)
>>>
>> I don't think there's a problem.
>>
>> First, the EOI message is itself asynchronous. While the write to
>> the local APIC is synchronous, effects on the rest of the system are
>> effected using an APIC message, which travels asynchronously.
>>
>> Second, a component that needs timely information doesn't have to
>> wait; it can read the eventfd and be sure it has seen all EOIs up to
>> now.
>>
>>
> I remember we already discussed the use of eventfd for reporting EOI and
> decided against it, but I don't remember why. :( Was it because if we
> are going to export EOI to userspace anyway we want to be able to use it
> for RTC timedrift fixing and for that we need to know what CPU called
> EOI and eventfd can't provide that?
>

IIRC it was the synchronity argument. But it's bogus: if the RTC wants
to know whether an ack occured before it makes some decision, all it has
to do is read() the eventfd and find out.

Another issue is which cpu issued the ack. I suppose we can have
per-vcpu eventfds, though that's ugly.

--
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