linux-kernel - Re: [PATCH v2 15/15] x86/lib/memcpy_64.S: Convert memcpy to ALTERNATIVE

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Message-ID: <20150305002640.GA20449@gmail.com>
Date:	Thu, 5 Mar 2015 01:26:40 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	X86 ML <x86@...nel.org>, Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 15/15] x86/lib/memcpy_64.S: Convert memcpy to
 ALTERNATIVE_2 macro


* Borislav Petkov <bp@...en8.de> wrote:

> On Wed, Mar 04, 2015 at 08:26:33AM +0100, Ingo Molnar wrote:
> > Since most CPUs we care about have ERMS, wouldn't it be better to
> > patch in the actual memcpy_erms sequence into the primary memcpy()
> > function? It's just about 9 bytes AFAICT.
> 
> Actually, most set REP_GOOD - all Intel family 6 and all relevant 
> AMDs.
> 
> And only the newer Intels have ERMS. My SNB, for example doesn't 
> while IVB has it. So I'd guess everything >= IVB would have it.

Well, my point equally applies to all variants: it's better to avoid 
the NOP or JMP overhead (however small it may be), by simply copying 
the ideal memcpy routine into memcpy()?

I.e. while I'd not want to patch in memcpy_orig (it's legacy really), 
but the other two variants, ERMS and REP MOVSQ could be patched in 
directly via ALTERNATIVE_2()?

Thanks,

	Ingo
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