linux-kernel - RE: [tip:x86/asm] x86/xor: Make virtualization friendly

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Message-ID: <a725c7f649e142c7a4063abaaaa6f1d2@SN2PR03MB061.namprd03.prod.outlook.com>
Date:	Sat, 26 Jan 2013 16:49:56 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"JBeulich@...e.com" <JBeulich@...e.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [tip:x86/asm] x86/xor: Make virtualization friendly



> -----Original Message-----
> From: H. Peter Anvin [mailto:hpa@...or.com]
> Sent: Friday, January 25, 2013 8:05 PM
> To: H. Peter Anvin
> Cc: mingo@...nel.org; linux-kernel@...r.kernel.org; konrad.wilk@...cle.com;
> torvalds@...ux-foundation.org; JBeulich@...e.com; tglx@...utronix.de; linux-
> tip-commits@...r.kernel.org; Marcelo Tosatti; KY Srinivasan; Haiyang Zhang
> Subject: Re: [tip:x86/asm] x86/xor: Make virtualization friendly
> 
> 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@...e.com>
> >>> AuthorDate: Fri, 2 Nov 2012 14:21:23 +0000
> >>> Committer:  Ingo Molnar <mingo@...nel.org>
> >>> 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?

Checking with the Hyper-V developers, Hyper-V does not have performance issues
With CR0.TS

Regards,

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