Re: [PATCH] KVM: SVM: Disable AVIC on SNP-enabled system without HvInUseWrAllowed feature

["Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>]

Hi Joao,

On 10/1/2024 6:04 PM, Joao Martins wrote:
> On 30/09/2024 06:50, Suravee Suthikulpanit wrote:
>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>> index dd4682857c12..921b6de80e24 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -448,6 +448,7 @@
>>   #define X86_FEATURE_SME_COHERENT	(19*32+10) /* AMD hardware-enforced cache coherency */
>>   #define X86_FEATURE_DEBUG_SWAP		(19*32+14) /* "debug_swap" AMD SEV-ES full debug state swap support */
>>   #define X86_FEATURE_SVSM		(19*32+28) /* "svsm" SVSM present */
>> +#define X86_FEATURE_HV_INUSE_WR_ALLOWED	(19*32+30) /* Write to in-use hypervisor-owned pages allowed */
>>   
>>   /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
>>   #define X86_FEATURE_NO_NESTED_DATA_BP	(20*32+ 0) /* No Nested Data Breakpoints */
>> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
>> index 4b74ea91f4e6..42f2caf17d6a 100644
>> --- a/arch/x86/kvm/svm/avic.c
>> +++ b/arch/x86/kvm/svm/avic.c
>> @@ -1199,6 +1199,12 @@ bool avic_hardware_setup(void)
>>   		return false;
>>   	}
>>   
>> +	if (cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
>> +	    !boot_cpu_has(X86_FEATURE_HV_INUSE_WR_ALLOWED)) {
>> +		pr_warn("AVIC disabled: missing HvInUseWrAllowed on SNP-enabled system");
>> +		return false;
>> +	}
>> +
> 
> Wouldn't be better to make this is APICv inhibit to allow non-SNP guests to work
> with AVIC?

I was considering the APICV inhibit as well, and decided to go with 
disabling AVIC since it does not require additional 
APICV_INHIBIT_REASON_XXX flag, and we can simply disable AVIC support 
during kvm-amd driver initialization.

After rethink this, it is better to use per-VM APICv inhibition instead 
since certain AVIC data structures will be needed for secure AVIC 
support in the future.

I will update this patch and send out V2.

Thanks,
Suravee