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

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Date:	Wed, 07 Jan 2015 06:41:58 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Marcelo Tosatti <mtosatti@...hat.com>
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>
Subject: Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso
 pvclock reader



On 06/01/2015 19:26, Andy Lutomirski wrote:
> Don't you stil need:
> 
> version++;
> write the rest;
> version++;
> 
> with possible smp_wmb() in there to keep the compiler from messing around?

No, see my other reply.

Separating the version write is a real bug, but that should be all that
it's needed.

> Also, if you do this, can you also make setting and clearing
> STABLE_BIT properly atomic across all vCPUs?  Or at least do something
> like setting it last and clearing it first on vPCU 0?

That would be nice if you want to make the pvclock area fit in a single
page.  However, it would have to be a separate flag bit, or a separate
CPUID feature.

Paolo
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