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

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Date:	Wed, 20 Aug 2014 18:01:38 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...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

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.
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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