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

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Date:	Wed, 20 Aug 2014 18:26:54 +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 18:03+0200, Paolo Bonzini:
> 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.

Oh, seqlock readers do not have to write to shared memory, so it is
acceptable ...

>                                         I don't like code that is
> unsafe by design...

I wouldn't say it is unsafe, because VCPU's PW is always greater than
module's PW. We are just going to PLE exit sooner than expected.
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