From:   "Michael Kelley (LINUX)" <mikelley@microsoft.com>
To:     Juergen Gross <jgross@suse.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>
CC:     "lists@nerdbynature.de" <lists@nerdbynature.de>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: RE: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs
Thread-Topic: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs
Thread-Index: AQHZPFdKu6v3b+HaGE2yB6PHNlodO67LulNA
Date:   Mon, 13 Feb 2023 01:07:47 +0000
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References: <20230209072220.6836-1-jgross@suse.com>
 <20230209072220.6836-3-jgross@suse.com>
In-Reply-To: <20230209072220.6836-3-jgross@suse.com>
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From: Juergen Gross <jgross@suse.com> Sent: Wednesday, February 8, 2023 11:=
22 PM
>=20
> When running virtualized, MTRR access can be reduced (e.g. in Xen PV
> guests or when running as a SEV-SNP guest under Hyper-V). Typically
> the hypervisor will reset the MTRR feature in cpuid data, resulting
> in no MTRR memory type information being available for the kernel.
>=20
> This has turned out to result in problems:
>=20
> - Hyper-V SEV-SNP guests using uncached mappings where they shouldn't
> - Xen PV dom0 mapping memory as WB which should be UC- instead
>=20
> Solve those problems by supporting to set a fixed MTRR state,
> overwriting the empty state used today. In case such a state has been
> set, don't call get_mtrr_state() in mtrr_bp_init(). The set state
> will only be used by mtrr_type_lookup(), as in all other cases
> mtrr_enabled() is being checked, which will return false. Accept the
> overwrite call only in case of MTRRs being disabled in cpuid.
>=20
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - new patch
> ---
>  arch/x86/include/asm/mtrr.h        |  2 ++
>  arch/x86/kernel/cpu/mtrr/generic.c | 38 ++++++++++++++++++++++++++++++
>  arch/x86/kernel/cpu/mtrr/mtrr.c    |  9 +++++++
>  3 files changed, 49 insertions(+)
>=20
> diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
> index f0eeaf6e5f5f..0b8f51d683dc 100644
> --- a/arch/x86/include/asm/mtrr.h
> +++ b/arch/x86/include/asm/mtrr.h
> @@ -31,6 +31,8 @@
>   */
>  # ifdef CONFIG_MTRR
>  void mtrr_bp_init(void);
> +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_v=
ar,
> +			  mtrr_type *fixed, mtrr_type def_type);

Could you add a stub for the !CONFIG_MTRR case?  Then the
#ifdef CONFIG_MTRR could be removed in Patch 3 of this series.

Michael