Re: [RESUBMIT][PATCH] x86/mm: Fix PAT bit missing from page protection modify mask

From: Juergen Gross
Date: Fri Jun 02 2023 - 10:48:20 EST

On 02.06.23 16:43, Borislav Petkov wrote:
On Thu, Jun 01, 2023 at 10:47:39AM +0200, Juergen Gross wrote:
As described in the commit message, this only works on bare metal due to the
PAT bit not being needed for WC mappings.

Making this patch Xen specific would try to cure the symptoms without fixing
the underlying problem: _PAGE_PAT should be regarded the same way as the bits
for caching mode (_PAGE_CHG_MASK).

So why isn't _PAGE_PAT part of _PAGE_CHG_MASK?

This would result in problems for large pages: _PAGE_PSE is at the same
position as _PAGE_PAT (large pages are using _PAGE_PAT_LARGE instead).

Yes, x86 ABI is a mess.


Juergen

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