Re: [PATCH v2 0/4] Fix accessed bit tracking

[Avi Kivity <avi@redhat.com>]

On 06/08/2010 05:35 AM, Xiao Guangrong wrote:
>
>> We can avoid the exchange in most cases, for example if the new spte has
>> the accessed bit set (already in the patch set) or if the page is
>> already marked as accessed, or if we see the old spte has the accessed
>> bit set (so no race can occur).  I'll update the patches to avoid
>> atomics when possible.
>>      
> Umm, the reason that we need atomics here is to avoid vcpu to update spte when we read A bit
> form it, so, perhaps we can use below way to avoid atomics completely:
>
> - set reserved bit in spte
> - get A bit form spte
> - set new spte
>
> the worst case is cause vcpu #PF here, but it doesn't matter since the old mapping is already invalid,
> also need a remote tlb flush later.
>    

To set the reserved bit in the spte, you need an atomic operation (well, 
unless you use a sub-word-acccess to set a reserved bit in the high 32 
bits).

>> I don't think atomics are that expensive, though, ~20 cycles on modern
>> processors?
>>
>>      
> Yes, but atomics are "LOCK" instructions, it can stop multiple cpus runing in parallel.
>    

Only if those cpus are accessing the same word you're accessing.

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