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

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Message-ID: <4BCC34DF.6030702@redhat.com>
Date:	Mon, 19 Apr 2010 13:47:59 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

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

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