From: Juergen Gross <jgross@suse.com> Sent: Wednesday, February 8, 2023 11:22 PM
>
> In order to avoid mappings using the UC- cache attribute, set the
> MTRR state to use WB caching as the default.
>
> This is needed in order to cope with the fact that PAT is enabled,
> while MTRRs are disabled by the hypervisor.
>
> Fixes: 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - new patch
> ---
> arch/x86/kernel/cpu/mshyperv.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 46668e255421..51e47dc0e987 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -34,6 +34,7 @@
> #include <clocksource/hyperv_timer.h>
> #include <asm/numa.h>
> #include <asm/coco.h>
> +#include <asm/mtrr.h>
>
> /* Is Linux running as the root partition? */
> bool hv_root_partition;
> @@ -335,6 +336,13 @@ static void __init ms_hyperv_init_platform(void)
> static_branch_enable(&isolation_type_snp);
> #ifdef CONFIG_SWIOTLB
> swiotlb_unencrypted_base = ms_hyperv.shared_gpa_boundary;
> +#endif
Unfortunately, Hyper-V does not filter out the MTRR flag from the
CPUID leaf, so this code also needs
setup_clear_cpu_cap(X86_FEATURE_MTRR);
before calling mtrr_overwrite_state(). I've got a bug filed for the
Hyper-V team to fix the flag, but clearing the feature here solves the
problem regardless.
> +#ifdef CONFIG_MTRR
Hopefully this #ifdef can go away, per my comment in Patch 2 of
the series.
Michael
> + /*
> + * Set WB as the default cache mode in case MTRRs are
> + * disabled by the hypervisor.
> + */
> + mtrr_overwrite_state(NULL, 0, NULL, MTRR_TYPE_WRBACK);
> #endif
> }
> /* Isolation VMs are unenlightened SEV-based VMs, thus this check: */
> --
> 2.35.3