Re: [PATCH 1/5] Add a global synchronization point for pvclock

Avi Kivity
2010-04-19T10:47:59Z

On 04/19/2010 01:43 PM, Peter Zijlstra wrote:
>
>>>
>>>> +
>>>> cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
>>>> {
>>>> struct pvclock_shadow_time shadow;
>>>> unsigned version;
>>>> cycle_t ret, offset;
>>>> + u64 last;
>>>>
>>>>
>>>> + do {
>>>> + last = last_value;
>>>>
>>> Otherwise, this assignment can see a partial update.
>>>
>> On a 32-bit guest, that is.
>>
> Right, do bear in mind that the x86 implementation of atomic64_read() is
> terrifyingly expensive, it is better to not do that read and simply use
> the result of the cmpxchg.
>
>

atomic64_read() _is_ cmpxchg64b. Are you thinking of some clever
implementation for smp i386?

--
error compiling committee.c: too many arguments to function