Re: gettimeofday "slow" in RHEL4 guests

[Avi Kivity <avi@redhat.com>]

Dor Laor wrote:
>>>  
>>> +    if (atomic_read(&pt->pending) > 1)
>>> +        atomic_set(&pt->pending, 1);
>>> +
>>>   
>>
>> Replace the atomic_inc() with atomic_set(, 1) instead? One less test, 
>> and more important, the logic is scattered less around the source.
> But having only a pending bit instead of a counter will cause kvm to 
> drop pit irqs on rare high load situations.
> The disable reinjection option is better.

Both variants disable reinjection.  Forcing a counter to 1 every time it 
exceeds 1 is equivalent to maintaining a bit.

In both variants, there is a missing 'if (disable_reinjection)' (Marcelo 
mentioned this in the original message).

> Except for these 'tsc compensate' guest, what are the occasions where 
> the guest writes his tsc?
> If this is the only case we can disable reinjection once we trap tsc 
> writes.

I don't think these guests write to the tsc.  Rather, they read the tsc 
and the pit counters and try to correlate.  And fail.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.