Re: [PATCH v2 05/14] KVM: x86: Override reported SME/SEV feature flags with host mask

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/01/21 18:09, Sean Christopherson wrote:
> On Thu, Jan 28, 2021, Paolo Bonzini wrote:
>> On 14/01/21 01:36, Sean Christopherson wrote:
>>> Add a reverse-CPUID entry for the memory encryption word, 0x8000001F.EAX,
>>> and use it to override the supported CPUID flags reported to userspace.
>>> Masking the reported CPUID flags avoids over-reporting KVM support, e.g.
>>> without the mask a SEV-SNP capable CPU may incorrectly advertise SNP
>>> support to userspace.
>>>
>>> Cc: Brijesh Singh <brijesh.singh@amd.com>
>>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>> ---
>>>    arch/x86/kvm/cpuid.c | 2 ++
>>>    arch/x86/kvm/cpuid.h | 1 +
>>>    2 files changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index 13036cf0b912..b7618cdd06b5 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -855,6 +855,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>>>    	case 0x8000001F:
>>>    		if (!boot_cpu_has(X86_FEATURE_SEV))
>>>    			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>>> +		else
>>> +			cpuid_entry_override(entry, CPUID_8000_001F_EAX);
>>>    		break;
>>>    	/*Add support for Centaur's CPUID instruction*/
>>>    	case 0xC0000000:
>>> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
>>> index dc921d76e42e..8b6fc9bde248 100644
>>> --- a/arch/x86/kvm/cpuid.h
>>> +++ b/arch/x86/kvm/cpuid.h
>>> @@ -63,6 +63,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
>>>    	[CPUID_8000_0007_EBX] = {0x80000007, 0, CPUID_EBX},
>>>    	[CPUID_7_EDX]         = {         7, 0, CPUID_EDX},
>>>    	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
>>> +	[CPUID_8000_001F_EAX] = {0x8000001f, 1, CPUID_EAX},
>>>    };
>>>    /*
>>>
>>
>> I don't understand, wouldn't this also need a kvm_cpu_cap_mask call
>> somewhere else?  As it is, it doesn't do anything.
> 
> Ugh, yes, apparently I thought the kernel would magically clear bits it doesn't
> care about.
> 
> Looking at this again, I think the kvm_cpu_cap_mask() invocation should always
> mask off X86_FEATURE_SME.  SME cannot be virtualized, and AFAIK it's not
> emulated by KVM.  This would fix an oddity where SME would be advertised if SEV
> is also supported.
> 
> Boris has queue the kernel change to tip/x86/cpu, I'll spin v4 against that.

You can send it after the 5.12 merge window.

Paolo