linux-kernel - Re: 2.6.26-rc1 lies about PAT not being available

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Message-ID: <4820767D.8040202@zytor.com>
Date:	Tue, 06 May 2008 08:17:17 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mikael Pettersson <mikpe@...uu.se>, tglx@...utronix.de,
	mingo@...hat.com, venkatesh.pallipadi@...el.com,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.26-rc1 lies about PAT not being available

Ingo Molnar wrote:
>> I know about Intel's PAT erratum (only 4 PAT entries work) and 
>> understand that the kernel might not want to use a half-broken PAT 
>> implementation, but the statement above is blatantly false: the kernel 
>> IS built with PAT support and the CPU DOES have (half-broken) PAT HW 
>> support as /proc/cpuinfo from earlier kernels confirms.
> 
> well the code is just being overly conservative, and the message is 
> right: the kernel does not support PAT on that hw. Yet.
> 
> Please send us a patch and test whether it's working on your box? That's 
> how the PAT code evolved: it initially was only enabled on a very small 
> subset of CPUs. We can enable it on more CPUs if people test it.
> 
> To be conservative, please make the code emit a printk that this is PAT 
> on not fully-working hw platform (only 4 PAT entries work). If there 
> _is_ some erratum on that CPU affecting PAT reliability down the line, 
> we at least want to have some clue in the syslog.
> 

For what it's worth, we only use four PAT entries, so it doesn't matter 
that only four work.

	-hpa

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