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

[Juergen Gross <jgross@suse.com>]

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On 13.02.23 12:39, 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@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(+)
>>
>> 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.
> 
>> +		return;
> 
> So this here needs to check:
> 
> 	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR) &&
> 	    !(cpu_feature_enabled(X86_FEATURE_SEV_SNP) ||
> 	      cpu_feature_enabled(X86_FEATURE_XENPV))) {
> 		WARN_ON_ONCE(1);
> 		return;
> 	}
> 
> as we don't want this to be called somewhere or by something else.
> 
> The SEV_SNP flag can be used from:
> 
> https://lore.kernel.org/r/20221214194056.161492-14-michael.roth@amd.com
> 
> I'm assuming here HyperV SEV-SNP guests really do set that feature flag
> (they better). We can expedite that patch ofc.

Is that flag _really_ meant to indicate we are running as a SEV-SNP guest?

Given that the referenced patch is part of the SEV-SNP host support series,
I'm inclined to suspect it won't be set for sure in HyperV SEV-SNP guests.
And who is setting it for KVM SEV-SNP guests?


Juergen

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