linux-kernel - Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

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Date:	Fri, 09 Oct 2009 17:24:30 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	kurt.hackel@...cle.com, the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Keir Fraser <keir.fraser@...citrix.com>,
	Zach Brown <zach.brown@...cle.com>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

On 10/07/09 03:25, Avi Kivity wrote:
> def try_pvclock_vtime():
>   tsc, p0 = rdtscp()
>   v0 = pvclock[p0].version
>   tsc, p = rdtscp()
>   t = pvclock_time(pvclock[p], tsc)
>   if p != p0 or pvclock[p].version != v0:
>      raise Exception("Processor or timebased change under our feet")
>   return t

This doesn't quite work.

If we end up migrating some time after the first rdtscp, then the
accesses to pvclock[] will be cross-cpu.  Since we don't made any strong
SMP memory ordering guarantees on updating the structure, the snapshot
isn't guaranteed to be consistent even if we re-check the version at the
end.

So to use rdtscp we need to either redefine the update of
pvclock_vcpu_time_info to be SMP-safe, or keep the additional migration
check.

    J
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