linux-kernel - Re: [PATCH 2/9] x86/kernel/cpu/intel.c: Initialize Enhanced REP MOVSB/STOSBenhanced

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Message-ID: <ca1b13fa72e83adbc23b56e0cb28df7a.squirrel@www.firstfloor.org>
Date:	Wed, 18 May 2011 04:46:43 +0200
From:	"Andi Kleen" <andi@...stfloor.org>
To:	"Fenghua Yu" <fenghua.yu@...el.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H Peter Anvin" <hpa@...or.com>,
	"Asit K Mallick" <asit.k.mallick@...el.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Avi Kivity" <avi@...hat.com>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Andi Kleen" <andi@...stfloor.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"Fenghua Yu" <fenghua.yu@...el.com>
Subject: Re: [PATCH 2/9] x86/kernel/cpu/intel.c: Initialize Enhanced REP
 MOVSB/STOSBenhanced

> From: Fenghua Yu <fenghua.yu@...el.com>
>
> If kernel intends to use enhanced REP MOVSB/STOSB, it must ensure
> IA32_MISC_ENABLE.Fast_String_Enable (bit 0) is set and CPUID.(EAX=07H,
> ECX=0H):
> EBX[bit 9] also reports 1.

I suspect the check at this place is not too useful because it will
only work for the BSP. For all others it's too late -- the patching
has already happened.

So either this is a problem and then it should be checked on all CPUs.
Or maybe not at all.

The problem is that the alternative patching currently relies on being
run early with no other CPUs. It has no race protections, support
for cross modification etc.

While it would be possible to fix that it would be quite complicated
I bet.

So I think it's better to just remove it unless it's a real problem
in the field.

-Andi

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