From mboxrd@z Thu Jan  1 00:00:00 1970
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Subject: Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum
Date: Wed, 20 Aug 2014 18:26:54 +0200
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Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Gleb Natapov <gleb@kernel.org>,
	Raghavendra KT <raghavendra.kt@linux.vnet.ibm.com>,
	Vinod Chegu <chegu_vinod@hp.com>, Hui-Zhi <hui-zhi.zhao@hp.com>
To: Paolo Bonzini <pbonzini@redhat.com>
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2014-08-20 18:03+0200, Paolo Bonzini:
> Il 20/08/2014 18:01, Radim Kr=C4=8Dm=C3=A1=C5=99 ha scritto:
> > 2014-08-20 17:34+0200, Paolo Bonzini:
> >> Il 20/08/2014 17:31, Radim Kr=C4=8Dm=C3=A1=C5=99 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 ch=
anges?
> >>  I guess you could use a seqlock.
> >=20
> > Yes, for example between a modification of ple_window
> >   new =3D min(old, PW_actual_max) * PW_grow
> > which gets compiled into something like this:
> >   1) tmp =3D min(old, PW_actual_max)
> >   2) new =3D tmp * PW_grow
> > and a write to increase PW_grow
> >   3) PW_actual_max =3D min(PW_max / new_PW_grow, PW_actual_max)
> >   4) PW_grow =3D new_PW_grow
> >   5) PW_actual_max =3D PW_max / new_PW_grow
> >=20
> > 3 and 4 can exectute between 1 and 2, which could overflow.
> >=20
> > I don't think they are important enough to warrant a significant
> > performance hit of locking.
>=20
> A seqlock just costs two memory accesses to the same (shared) cache l=
ine
> 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.