[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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