netdev - Re: oops/warning report for the week of November 26, 2008

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Message-ID: <492F1462.4020803@zytor.com>
Date:	Thu, 27 Nov 2008 13:42:58 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	Ingo Molnar <mingo@...e.hu>, Yinghai Lu <yinghai@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	NetDev <netdev@...r.kernel.org>, x86@...nel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	jesse Barnes <jbarnes@...tuousgeek.org>
Subject: Re: oops/warning report for the week of November 26, 2008

Arjan van de Ven wrote:
> 
> diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
> index 1159e26..0044e61 100644
> --- a/arch/x86/kernel/cpu/mtrr/main.c
> +++ b/arch/x86/kernel/cpu/mtrr/main.c
> @@ -1567,6 +1567,8 @@ int __init mtrr_trim_uncached_memory(unsigned long end_pfn)
>  	 * Make sure we only trim uncachable memory on machines that
>  	 * support the Intel MTRR architecture:
>  	 */
> +	if (!cpu_has_mtrr)
> +		return 0;
>  	if (!is_cpu(INTEL) || disable_mtrr_trim)
>  		return 0;
>  	rdmsr(MTRRdefType_MSR, def, dummy);

Okay... is_cpu() here is defined as:

#define is_cpu(vnd)      (mtrr_if && mtrr_if->vendor == X86_VENDOR_##vnd)

... so an MTRR interface has been identified.  Therefore testing 
cpu_has_mtrr is redundant.

As far as use_intel() versus is_cpu(INTEL), it looks to me as though the 
two are identical in the current code -- mtrr_if->vendor is never set in 
the generic code, and so defaults to 0 - meaning X86_VENDOR_INTEL.

All in all, it looks like the vendor ID stuff is a bad case of "works by 
accident" in the MTRR code, however, *given the current code* I conclude 
that is_cpu(INTEL) == use_intel() and that neither can be true without 
MTRRs enabled.

	-hpa
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