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

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Message-ID: <afc57813-b94d-4dfd-a45d-96204edac1e9@email.android.com>
Date:	Mon, 28 Jan 2013 07:26:59 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	mingo@...nel.org, tglx@...utronix.de,
	torvalds@...ux-foundation.org, konrad.wilk@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [tip:x86/asm] x86/xor: Make virtualization friendly

It adds substantial boot time, and it has no value when the cache priority rules force the non-cache-polluting version even if somewhat slower... which can and does happen.

Jan Beulich <JBeulich@...e.com> wrote:

>>>> On 25.01.13 at 23:11, "H. Peter Anvin" <hpa@...or.com> 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.
>
>I don't follow: The patch doesn't penalize anyone, it merely
>widens the set of methods tried on virtualized platforms. I.e.
>if other hypervisors have no problem here, then the best
>performing one should still turn out to be the SSE or AVX one.
>Or if it doesn't, it ought to be to their advantage (I would even
>question why this extra probing isn't done on native too, e.g.
>to cope with eventual bad vector implementations, say on
>low-power/low-cost CPUs).
>
>Jan

-- 
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