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

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Message-ID: <51032BCB.5070506@zytor.com>
Date:	Fri, 25 Jan 2013 17:05:15 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"H. Peter Anvin" <hpa@...or.com>
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 <mtosatti@...hat.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
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?

	-hpa

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

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