Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT

["H. Peter Anvin" <hpa@zytor.com>]

Keir Fraser wrote:
> 
> My suggestion is that Xen changes its PAT[1] from WT to WC. AFAIK only
> suse's Linux port is currently using Xen's PAT support, and it does not
> actually use the WT cache attribute. Then Xen's PAT[0-3] would match what
> Linux expects, and Linux apparently does not risk using PAT[4-7] because of
> processor errata so their values are irrelevant. Linux can check that it is
> running on Xen with suitable PAT setup by reading MSR_IA32_CR_PAT and
> checking PAT[0-3], and only enable PAT usage in case of a match.
> 

There exists at least one processor erratum where the CPU will use
PAT[4-7] when the user requested PAT[0-3].  For those CPUs, it is unsafe
for *any* OS to have PAT[4-7] != PAT[0-3].

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.