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Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode availability
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 4 May 2022 11:51:17 +0200
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- Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>, Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>, Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>, Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>, David Airlie <airlied@xxxxxxxx>, Daniel Vetter <daniel@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 04 May 2022 09:51:29 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04.05.2022 11:14, Juergen Gross wrote:
> On 04.05.22 10:31, Jan Beulich wrote:
>> On 03.05.2022 15:22, Juergen Gross wrote:
>>> Some drivers are using pat_enabled() in order to test availability of
>>> special caching modes (WC and UC-). This will lead to false negatives
>>> in case the system was booted e.g. with the "nopat" variant and the
>>> BIOS did setup the PAT MSR supporting the queried mode, or if the
>>> system is running as a Xen PV guest.
>>
>> While, as per my earlier patch, I agree with the Xen PV case, I'm not
>> convinced "nopat" is supposed to honor firmware-provided settings. In
>> fact in my patch I did arrange for "nopat" to also take effect under
>> Xen PV.
>
> Depends on what the wanted semantics for "nopat" are.
>
> Right now "nopat" will result in the PAT MSR left unchanged and the
> cache mode translation tables be initialized accordingly.
>
> So does "nopat" mean that the PAT MSR shouldn't be changed, or that
> PAGE_BIT_PAT will never be set?
According to the documentation for the option ("Disable PAT (page
attribute table extension of pagetables) support") I'd say the latter.
Jan
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