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

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Message-ID: <4AD7754B.20701@goop.org>
Date:	Thu, 15 Oct 2009 12:17:31 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.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>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir.fraser@...citrix.com>,
	Zach Brown <zach.brown@...cle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock:	add	vsyscall	implementation

On 10/14/09 05:32, Avi Kivity wrote:
> On 10/14/2009 05:00 AM, Jeremy Fitzhardinge wrote:
>>
>>> So it's broken or disabled when that assumption is wrong?  We could
>>> easily fix that now.  Might even reuse the pvclock structures.
>>>      
>> Well, the kernel internally makes more or less the same assumption; the
>> vsyscall clocksource is the same as the kernel's internal one.  I think
>> idea is that it just drops back to something like hpet if the tsc
>> doesn't have very simple SMP characteristics.
>>
>> If the kernel could characterize the per-cpu properties of the tsc more
>> accurately, then it could use the pvclock mechanism on native.
>>    
>
> It does - that's how kvm implements pvclock on the host side.  See
> kvmclock_cpufreq_notifier() in arch/x86/kvm/x86.c.

The tsc clocksource currently seems a fair bit more picky though; it
doesn't attempt to sync tscs or work out their relative offsets or
rates.  Which seems a bit defeatist.

    J
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