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