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

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Message-ID: <54ACF5AC.4030700@redhat.com>
Date:	Wed, 07 Jan 2015 10:00:28 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>, Gleb Natapov <gleb@...nel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso
 pvclock reader



On 07/01/2015 08:18, Andy Lutomirski wrote:
>>> >> Thus far, I've been told unambiguously that a guest can't observe pvti
>>> >> while it's being written, and I think you're now telling me that this
>>> >> isn't true and that a guest *can* observe pvti while it's being
>>> >> written while the low bit of the version field is not set.  If so,
>>> >> this is rather strongly incompatible with the spec in the KVM docs.
>> >
>> > Where am I saying that?
> I thought the conclusion from what you and Marcelo pointed out about
> the code was that, once the first vCPU updated its pvti, it could
> start running guest code while the other vCPUs are still updating
> pvti, so its guest code can observe the other vCPUs mid-update.

Ah, in that sense you're right.  However, each VCPU cannot observe _its
own_ pvti entry while it's being written (no matter what's in the low
bit of the version field).

Paolo
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