linux-kernel - Re: [Xen-devel] [PATCH RFC 0/3] x86: Full support of PAT

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Message-Id: <53F4A0D202000078000BAD84@mail.emea.novell.com>
Date:	Wed, 20 Aug 2014 13:21:22 +0100
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<gnomes@...rguk.ukuu.org.uk>
Cc:	<stefan.bader@...onical.com>, <toshi.kani@...com>,
	<x86@...nel.org>, <ville.syrjala@...ux.intel.com>,
	<xen-devel@...ts.xensource.com>, "Juergen Gross" <JGross@...e.com>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH RFC 0/3] x86: Full support of PAT

>>> One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> 08/20/14 2:08 PM >>>
>> The Linux kernel currently supports only 4 different cache modes. The PAT MSR
>> is set up in a way that the setting of _PAGE_PAT in a pte doesn't matter: the
>> top 4 entries in the PAT MSR are the same as the 4 lower entries.
>> 
>> This results in the kernel not supporting e.g. write-through mode. Especially
>> this cache mode would speed up drivers of video cards which now have to use
>> uncached accesses.
>
>Pentium II erratum A52 (and similar on quite a few other processors)

But you're aware that for the last two major releases Xen hasn't been supporting
32-bit CPUs at all anymore? I.e. why should we limit functionality under Xen
based on errata on 64-bit-incapable CPUs?

Jan

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