Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum

From: Paolo Bonzini
Date: Wed Aug 20 2014 - 12:03:43 EST


Il 20/08/2014 18:01, Radim KrÄmÃÅ ha scritto:
> 2014-08-20 17:34+0200, Paolo Bonzini:
>> Il 20/08/2014 17:31, Radim KrÄmÃÅ ha scritto:
>>> Btw. without extra code, we are still going to overflow on races when
>>> changing PW_grow, should they be covered as well?
>>
>> You mean because there is no spinlock or similar protecting the changes?
>> I guess you could use a seqlock.
>
> Yes, for example between a modification of ple_window
> new = min(old, PW_actual_max) * PW_grow
> which gets compiled into something like this:
> 1) tmp = min(old, PW_actual_max)
> 2) new = tmp * PW_grow
> and a write to increase PW_grow
> 3) PW_actual_max = min(PW_max / new_PW_grow, PW_actual_max)
> 4) PW_grow = new_PW_grow
> 5) PW_actual_max = PW_max / new_PW_grow
>
> 3 and 4 can exectute between 1 and 2, which could overflow.
>
> I don't think they are important enough to warrant a significant
> performance hit of locking.

A seqlock just costs two memory accesses to the same (shared) cache line
as the PW data, and a non-taken branch. I don't like code that is
unsafe by design...

Paolo

> Or even more checks that would prevent it in a lockless way.
>
> (I'd just see that the result is set to something legal and also drop
> line 3, because it does not help things that much.)
>

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