linux-kernel - Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software defined MTRRs

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Date:   Thu, 16 Feb 2023 03:07:28 -0800
From:   Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, lists@...dbynature.de, mikelley@...rosoft.com,
        torvalds@...ux-foundation.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs

On Mon, Feb 13, 2023 at 12:39:40PM +0100, Borislav Petkov wrote:
> On Thu, Feb 09, 2023 at 08:22:14AM +0100, Juergen Gross wrote:
> > 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.
> > 
> > This has turned out to result in problems:
> > 
> > - Hyper-V SEV-SNP guests using uncached mappings where they shouldn't
> > - Xen PV dom0 mapping memory as WB which should be UC- instead
> > 
> > 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.
> 
> s/cpuid/CPUID/g
> 
> > Signed-off-by: Juergen Gross <jgross@...e.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(+)
> > 
> > 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_var,
> > +			  mtrr_type *fixed, mtrr_type def_type);
> >  extern u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform);
> >  extern void mtrr_save_fixed_ranges(void *);
> >  extern void mtrr_save_state(void);
> > diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> > index ee09d359e08f..788bc16888a5 100644
> > --- a/arch/x86/kernel/cpu/mtrr/generic.c
> > +++ b/arch/x86/kernel/cpu/mtrr/generic.c
> > @@ -240,6 +240,44 @@ static u8 mtrr_type_lookup_variable(u64 start, u64 end, u64 *partial_end,
> >  	return mtrr_state.def_type;
> >  }
> >  
> > +/**
> > + * mtrr_overwrite_state - set fixed MTRR state
> 
> fixed only? You pass in variable too...
> 
> > + *
> > + * Used to set MTRR state via different means (e.g. with data obtained from
> > + * a hypervisor).
> > + */
> > +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> > +			  mtrr_type *fixed, mtrr_type def_type)
> > +{
> > +	unsigned int i;
> > +
> > +	if (boot_cpu_has(X86_FEATURE_MTRR))
> 
> check_for_deprecated_apis: WARNING: arch/x86/kernel/cpu/mtrr/generic.c:254: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.
> 

Hi Boris,

Where does this check come from? I can't find a source for it.

Jeremi