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

[Juergen Gross]

On 02.06.23 16:48, Juergen Gross wrote:
> 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.

Oh, wait: I originally thought _PAGE_CHG_MASK would be used for large pages,
too. There is _HPAGE_CHG_MASK for that purpose.

So adding _PAGE_PAT to _PAGE_CHG_MASK and _PAGE_PAT_LARGE to _HPAGE_CHG_MASK
should do the job. At least I hope so.


Juergen

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