linux-kernel - Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

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Message-ID: <54AB9FFD.6070309@redhat.com>
Date:	Tue, 06 Jan 2015 09:42:37 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
CC:	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso
 pvclock reader



On 05/01/2015 23:48, Marcelo Tosatti wrote:
>>> > > But there is no guarantee that vCPU-N has updated its pvti when
>>> > > vCPU-M resumes guest instruction execution.
>> > 
>> > Still confused.  So we can freeze all vCPUs in the host, then update
>> > pvti 1, then resume vCPU 1, then update pvti 0?  In that case, we have
>> > a problem, because vCPU 1 can observe pvti 0 mid-update, and KVM
>> > doesn't increment the version pre-update, and we can return completely
>> > bogus results.
> Yes.

But then the getcpu test would fail (1->0).  Even if you have an ABA
situation (1->0->1), it's okay because the pvti that is fetched is the
one returned by the first getcpu.

Paolo
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