Re: [tip:x86/asm] x86/xor: Make virtualization friendly

[H. Peter Anvin]

On 01/25/2013 02:15 PM, H. Peter Anvin wrote:
> On 01/25/2013 02:11 PM, H. Peter Anvin wrote:
> > On 01/25/2013 02:43 AM, tip-bot for Jan Beulich wrote:
> > > Commit-ID:  05fbf4d6fc6a3c0c3e63b77979c9311596716d10
> > > Gitweb:     http://git.kernel.org/tip/05fbf4d6fc6a3c0c3e63b77979c9311596716d10
> > > Author:     Jan Beulich <JBeulich@xxxxxxxx>
> > > AuthorDate: Fri, 2 Nov 2012 14:21:23 +0000
> > > Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> > > CommitDate: Fri, 25 Jan 2013 09:23:51 +0100
> > >
> > > x86/xor: Make virtualization friendly
> > >
> > > In virtualized environments, the CR0.TS management needed here
> > > can be a lot slower than anticipated by the original authors of
> > > this code, which particularly means that in such cases forcing
> > > the use of SSE- (or MMX-) based implementations is not desirable
> > > - actual measurements should always be done in that case.
> > >
> > > For consistency, pull into the shared (32- and 64-bit) header
> > > not only the inclusion of the generic code, but also that of the
> > > AVX variants.
> >
> > This patch is wrong and should be dropped.  I verified it with the KVM
> > people that they do NOT want this change.  It is a Xen-specific problem.
>
> FWIW: I have dropped this patch from tip:x86/asm.

The bottom line, I guess, is that we need something like 
cpu_has_slow_kernel_fpu or something like that, and set it for 
specifically affected hypervisors?

Do we know if Hyper-V has performance issues with CR0.TS?

	-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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