Re: [PATCH v4 05/12] x86/xen: set MTRR state when running as Xen PV initial domain

[Boris Ostrovsky]

On 3/6/23 11:34 AM, Juergen Gross wrote:
> When running as Xen PV initial domain (aka dom0), MTRRs are disabled
> by the hypervisor, but the system should nevertheless use correct
> cache memory types. This has always kind of worked, as disabled MTRRs
> resulted in disabled PAT, too, so that the kernel avoided code paths
> resulting in inconsistencies. This bypassed all of the sanity checks
> the kernel is doing with enabled MTRRs in order to avoid memory
> mappings with conflicting memory types.
>
> This has been changed recently, leading to PAT being accepted to be
> enabled, while MTRRs stayed disabled. The result is that
> mtrr_type_lookup() no longer is accepting all memory type requests,
> but started to return WB even if UC- was requested. This led to
> driver failures during initialization of some devices.
>
> In reality MTRRs are still in effect, but they are under complete
> control of the Xen hypervisor. It is possible, however, to retrieve
> the MTRR settings from the hypervisor.
>
> In order to fix those problems, overwrite the MTRR state via
> mtrr_overwrite_state() with the MTRR data from the hypervisor, if the
> system is running as a Xen dom0.
>
> Fixes: 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen")
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>