Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Jan Beulich]

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