RE: [PATCH v5 05/15] x86/hyperv: set MTRR state when running as SEV-SNP Hyper-V guest

[Michael Kelley (LINUX)]

From: Juergen Gross <jgross@xxxxxxxx> Sent: Friday, March 31, 2023 11:37 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 not supported by the hypervisor.
> 
> Fixes: 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Tested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>

I've tested the full v5 series in a Hyper-V SEV-SNP guest with vTOM.  All looks
good.  The following output appears in dmesg, which I think matches expectations:

[    0.004000] MTRR map: 0 entries (0 fixed + 0 variable; max 0), built from 0 variable MTRRs
[    0.004000] MTRRs set to read-only
[    0.004000] x86/PAT: Configuration [0-7]: WB  WC  UC- UC  WB  WP  UC- WT

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

> ---
> V2:
> - new patch
> ---
>  arch/x86/kernel/cpu/mshyperv.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index f36dc2f796c5..0a6cc3cf8919 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;
> @@ -408,6 +409,9 @@ static void __init ms_hyperv_init_platform(void)
>  #ifdef CONFIG_SWIOTLB
>  			swiotlb_unencrypted_base =
> ms_hyperv.shared_gpa_boundary;
>  #endif
> +
> +			/* Set WB as the default cache mode. */
> +			mtrr_overwrite_state(NULL, 0, MTRR_TYPE_WRBACK);

The placement of this call needs to change due to a patch set I submitted and
that Boris has accepted into the 'tip' tree for the 6.4 merge window (so it's now
in linux-next).  The call should be placed in the function hv_vtom_init() in the
source file arch/x86/hyperv/ivm.c.  Anywhere in hv_vtom_init() is fine -- I would
suggest at the end.

Minor request: In a v6 of this patch set, please include me on all the patches in
the series so I can easily keep track of how it is progressing.

Michael

>  		}
>  		/* Isolation VMs are unenlightened SEV-based VMs, thus this check: */
>  		if (IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT)) {
> --
> 2.35.3