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[Xen-devel] Re: Live migration fails due to c/s 20627

To: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>
Subject: [Xen-devel] Re: Live migration fails due to c/s 20627
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 15 Dec 2009 09:12:00 -0800
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "kurt.hackel@xxxxxxxxxx" <kurt.hackel@xxxxxxxxxx>, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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On 12/15/09 08:10, Xu, Dongxiao wrote:
Why not just add the code to do rdtscp emulation,
which would NOT break live migration?
Add rdtscp emulation has such problem that, in Intel VMX, the
vmexit control for rdtsc and rdtscp is the same, so if we trap
rdtscp for emulation, OS will suffer from looooots of rdtsc vmexit,
which will bring performance downgrade.

I don't see why that's relevant. In the case where you've migrated the domain, if the CPU has rdtsc but not rdtscp, won't the rdtscp vmexit with an illegal instruction trap? In that case you can emulate rdtscp while still having direct execution of rdtsc.

Of course, having a wide difference between rdtscp and rdtsc performance may cause its own set of problems.


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