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

["Nakajima, Jun" <jun.nakajima@xxxxxxxxx>]

Dan Magenheimer wrote on Tue, 15 Dec 2009 at 09:08:40:

> 
> As Jeremy has pointed out, the guest OS already has
> other mechanisms to provide this information, and
> as Jun has pointed out, the non-rdtscp mechanism (lsl
> on Linux) may even be faster. Windows does not even
> provide TSC_AUX, so it definitely has other ways to
> obtain node/cpu info.  And, as I've said before,
> the node/cpu info provided by Linux in TSC_AUX is
> wrong anyway (except in very constrained environments
> such as where the admin has pinned vcpus to pcpus).

I think we should distinguish architectural support and Linux-specific issues. 
We need to enable RDSTCP support in HVM if user apps want to use it. If we want 
the Linux or other kernel to stop using TSC_AUX, we can mask off the feature in 
CPUID. Access to the MSR should result in #GP in the guest if the feature is 
not advertized. 

BTW, I'm hearing that the max TSC difference among CPUs is less than 10 cycles 
or so (virtually 0) at least Intel-based machines (except limited ones with 
known issues), so probably such pinning should not be required because vcpu 
migration should take more than.  

Jun
___
Intel Open Source Technology Center




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