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

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Message-ID: <4AD4DC52.8090508@goop.org>
Date:	Tue, 13 Oct 2009 13:00:18 -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>,
	Chris Mason <chris.mason@...cle.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add	vsyscall	implementation

On 10/12/09 23:39, Avi Kivity wrote:
> On 10/12/2009 09:13 PM, Jeremy Fitzhardinge wrote:
>> On 10/12/09 11:29, Avi Kivity wrote:
>>   
>>> Good catch.  Doesn't that invalidate rdtscp based vgettimeofday on
>>> non-virt as well (assuming p == cpu)?
>>>      
>> The tsc clocksource assumes the tsc is (mostly?) synced; it doesn't use
>> rdtscp or make any attempt at per-cpu corrections.
>>    
>
> 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.

    J

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