linux-kernel - Re: [PATCH] x86: Don't use NOPL on 32-bit cpu's because not all systems support it.

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Message-ID: <48CFDF82.3000704@zytor.com>
Date:	Tue, 16 Sep 2008 09:32:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	linux@...dersweb.net
CC:	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] x86: Don't use NOPL on 32-bit cpu's because not all systems
 support it.

David Sanders wrote:
> From: David Sanders <linux@...dersweb.net>
> 
> Currently in alternative.c NOPLs are introduced based on the synthetic
> cpu feature X86_FEATURE_NOPL.  However, some systems (like Virtual PC 2007)
> appear to support it but then will not boot 50% of the time because of
> the NOPLs (when paravirtualization support is built into kernel).
> This patch standardizes the treatment of NOPL to be like
> include/asm-x86/nops.h which only uses NOPLs on 64-bit processors.
> 

Oh good grief.

VPC is apparently so broken that these instructions work *some* of the 
time, which may include the first time, but not later.  That is an 
impressive level of cockup.

Given that, we should either just rip this code out, or detect VPC 
(how?).  The latter option can be done post-.27, of course; if so, we 
should just force the bit off for now rather than doing it in the 
alternatives code.  The only reason for doing the latter at all would be 
to mitigate the overhead of paravirt_ops and other dynamic patch sites.

It would also be the least impact for .27, I believe.

	-hpa

View attachment "0001-x86-completely-disable-NOPL-on-32-bits.patch" of type "text/x-patch" (2065 bytes)