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

[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
>>
>> 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@suse.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.

Okay, will add it.

> 
>> +#ifdef CONFIG_MTRR
> 
> Hopefully this #ifdef can go away, per my comment in Patch 2 of
> the series.

As said already, fine with me.


Juergen

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