Re: [PATCH 0/3] x86: make pat and mtrr independent from each other

[Chuck Zmudzinski <brchuckz@xxxxxxxxxxxx>]

On 7/15/2022 10:25 AM, Juergen Gross wrote:
> Today PAT can't be used without MTRR being available, unless MTRR is at
> least configured via CONFIG_MTRR and the system is running as Xen PV
> guest. In this case PAT is automatically available via the hypervisor,
> but the PAT MSR can't be modified by the kernel and MTRR is disabled.
>
> As an additional complexity the availability of PAT can't be queried
> via pat_enabled() in the Xen PV case, as the lack of MTRR will set PAT
> to be disabled. This leads to some drivers believing that not all cache
> modes are available, resulting in failures or degraded functionality.
>
> The same applies to a kernel built with no MTRR support: it won't
> allow to use the PAT MSR, even if there is no technical reason for
> that, other than setting up PAT on all cpus the same way (which is a
> requirement of the processor's cache management) is relying on some
> MTRR specific code.
>
> Fix all of that by:
>
> - moving the function needed by PAT from MTRR specific code one level
>   up
> - adding a PAT indirection layer supporting the 3 cases "no or disabled
>   PAT", "PAT under kernel control", and "PAT under Xen control"
> - removing the dependency of PAT on MTRR
>
> Juergen Gross (3):
>   x86: move some code out of arch/x86/kernel/cpu/mtrr
>   x86: add wrapper functions for mtrr functions handling also pat
>   x86: decouple pat and mtrr handling
>
>  arch/x86/include/asm/memtype.h     |  13 ++-
>  arch/x86/include/asm/mtrr.h        |  27 ++++--
>  arch/x86/include/asm/processor.h   |  10 +++
>  arch/x86/kernel/cpu/common.c       | 123 +++++++++++++++++++++++++++-
>  arch/x86/kernel/cpu/mtrr/generic.c |  90 ++------------------
>  arch/x86/kernel/cpu/mtrr/mtrr.c    |  58 ++++---------
>  arch/x86/kernel/cpu/mtrr/mtrr.h    |   1 -
>  arch/x86/kernel/setup.c            |  12 +--
>  arch/x86/kernel/smpboot.c          |   8 +-
>  arch/x86/mm/pat/memtype.c          | 127 +++++++++++++++++++++--------
>  arch/x86/power/cpu.c               |   2 +-
>  arch/x86/xen/enlighten_pv.c        |   4 +
>  12 files changed, 289 insertions(+), 186 deletions(-)
>

This patch series seems related to the regression reported
here on May 5, 2022:

https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/

I am experiencing that regression and could test this patch
on my system.

Can you confirm that with this patch series you are trying
to fix that regression?

Chuck