RE: [PATCH v6 00/16] x86/mtrr: fix handling with PAT but without MTRR

["Michael Kelley (LINUX)" <mikelley@xxxxxxxxxxxxx>]

From: Juergen Gross <jgross@xxxxxxxx> Sent: Tuesday, May 2, 2023 5:09 AM
> 
> This series tries to fix the rather special case of PAT being available
> without having MTRRs (either due to CONFIG_MTRR being not set, or
> because the feature has been disabled e.g. by a hypervisor).
> 
> The main use cases are Xen PV guests and SEV-SNP guests running under
> Hyper-V.
> 
> Instead of trying to work around all the issues by adding if statements
> here and there, just try to use the complete available infrastructure
> by setting up a read-only MTRR state when needed.
> 
> In the Xen PV case the current MTRR MSR values can be read from the
> hypervisor, while for the SEV-SNP case all needed is to set the
> default caching mode to "WB".
> 
> I have added more cleanup which has been discussed when looking into
> the most recent failures.
> 
> Note that I couldn't test the Hyper-V related change (patch 3).
> 
> Running on bare metal and with Xen didn't show any problems with the
> series applied.
> 
> It should be noted that patches 9+10 are replacing today's way to
> lookup the MTRR cache type for a memory region from looking at the
> MTRR register values to building a memory map with the cache types.
> This should make the lookup much faster and much easier to understand.
> 
> Changes in V2:
> - replaced former patches 1+2 with new patches 1-4, avoiding especially
>   the rather hacky approach of V1, while making all the MTRR type
>   conflict tests available for the Xen PV case
> - updated patch 6 (was patch 4 in V1)
> 
> Changes in V3:
> - dropped patch 5 of V2, as already applied
> - split patch 1 of V2 into 2 patches
> - new patches 6-10
> - addressed comments
> 
> Changes in V4:
> - addressed comments
> 
> Changes in V5
> - addressed comments
> - some other small fixes
> - new patches 3, 8 and 15
> 
> Changes in V6:
> - patch 1 replaces patches 1+2 of V5
> - new patches 8+12
> - addressed comments
> 
> Juergen Gross (16):
>   x86/mtrr: remove physical address size calculation
>   x86/mtrr: replace some constants with defines
>   x86/mtrr: support setting MTRR state for software defined MTRRs
>   x86/hyperv: set MTRR state when running as SEV-SNP Hyper-V guest
>   x86/xen: set MTRR state when running as Xen PV initial domain
>   x86/mtrr: replace vendor tests in MTRR code
>   x86/mtrr: have only one set_mtrr() variant
>   x86/mtrr: move 32-bit code from mtrr.c to legacy.c
>   x86/mtrr: allocate mtrr_value array dynamically
>   x86/mtrr: add get_effective_type() service function
>   x86/mtrr: construct a memory map with cache modes
>   x86/mtrr: add mtrr=debug command line option
>   x86/mtrr: use new cache_map in mtrr_type_lookup()
>   x86/mtrr: don't let mtrr_type_lookup() return MTRR_TYPE_INVALID
>   x86/mm: only check uniform after calling mtrr_type_lookup()
>   x86/mtrr: remove unused code
> 
>  .../admin-guide/kernel-parameters.txt         |   4 +
>  arch/x86/hyperv/ivm.c                         |   4 +
>  arch/x86/include/asm/mtrr.h                   |  43 +-
>  arch/x86/include/uapi/asm/mtrr.h              |   6 +-
>  arch/x86/kernel/cpu/mtrr/Makefile             |   2 +-
>  arch/x86/kernel/cpu/mtrr/amd.c                |   2 +-
>  arch/x86/kernel/cpu/mtrr/centaur.c            |  11 +-
>  arch/x86/kernel/cpu/mtrr/cleanup.c            |  22 +-
>  arch/x86/kernel/cpu/mtrr/cyrix.c              |   2 +-
>  arch/x86/kernel/cpu/mtrr/generic.c            | 677 ++++++++++++------
>  arch/x86/kernel/cpu/mtrr/legacy.c             |  90 +++
>  arch/x86/kernel/cpu/mtrr/mtrr.c               | 195 ++---
>  arch/x86/kernel/cpu/mtrr/mtrr.h               |  18 +-
>  arch/x86/kernel/setup.c                       |   2 +
>  arch/x86/mm/pgtable.c                         |  24 +-
>  arch/x86/xen/enlighten_pv.c                   |  52 ++
>  16 files changed, 721 insertions(+), 433 deletions(-)
>  create mode 100644 arch/x86/kernel/cpu/mtrr/legacy.c
> 
> --
> 2.35.3

I've tested the full v6 series in a normal Hyper-V guest and in an SEV-SNP 
guest.

In the SNP guest, the page attributes in /sys/kernel/debug/x86/pat_memtype_list
are "write-back" in the expected cases.  The "mtrr" x86 feature no longer 
appears
in the "flags" output of "lscpu" or /proc/cpuinfo.  /proc/mtrr does not exist, 
again
as expected.

In a normal VM, the "mtrr" x86 feature appears in the flags, and /proc/mtrr
shows expected values.  The boot option mtrr=debug works as expected.

Tested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>