linux-kernel - RE: [PATCH v2 3/8] x86/hyperv: set MTRR state when running as SEV-SNP Hyper-V guest

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Message-ID: <BYAPR21MB1688C2A893BA9D2A14D729DFD7DD9@BYAPR21MB1688.namprd21.prod.outlook.com>
Date:   Mon, 13 Feb 2023 01:07:58 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Juergen Gross <jgross@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     "lists@...dbynature.de" <lists@...dbynature.de>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: RE: [PATCH v2 3/8] x86/hyperv: set MTRR state when running as SEV-SNP
 Hyper-V guest

From: Juergen Gross <jgross@...e.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@...e.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