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

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Message-ID: <53F4C6D2.9050209@redhat.com>
Date:	Wed, 20 Aug 2014 18:03:30 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Gleb Natapov <gleb@...nel.org>,
	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>,
	Vinod Chegu <chegu_vinod@...com>, Hui-Zhi <hui-zhi.zhao@...com>
Subject: Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum

Il 20/08/2014 18:01, Radim Krčmář ha scritto:
> 2014-08-20 17:34+0200, Paolo Bonzini:
>> Il 20/08/2014 17:31, Radim Krčmář 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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