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 02:07, Michael Kelley (LINUX) wrote:
> From: Juergen Gross <jgross@suse.com> Sent: Wednesday, February 8, 2023 11:22 PM
>>
>> 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.
>>
>> 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);
> 
> Could you add a stub for the !CONFIG_MTRR case?  Then the
> #ifdef CONFIG_MTRR could be removed in Patch 3 of this series.

I was on the edge whether to add a stub. The Xen use case strongly
suggests that the code wants to be inside an #ifdef, while the Hyper-V
case is so simple, that it would benefit from the stub. As there was
another #ifdef just above the added code in mshyperv.c I believed it
would be fine without a stub. As you seem to like it better with the
stub, I can add it.


Juergen

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