Re: [PATCH] KVM: x86: Add KVM-only CPUID.0xC0000001:EDX feature bits

[Ewan Hai]

On 3/6/26 3:43 AM, Sean Christopherson wrote:
> 
> 
> On Thu, Mar 05, 2026, Ewan Hai wrote:
>> Per Paolo's suggestion, add the missing CPUID.0xC0000001:EDX feature
>> bits as KVM-only X86_FEATURE_* definitions, so KVM can expose them to
>> userspace before they are added to the generic cpufeatures definitions.
>>
>> Wire the new bits into kvm_set_cpu_caps() for CPUID_C000_0001_EDX.
>>
>> As a result, KVM_GET_SUPPORTED_CPUID reports these bits according to
>> host capability, allowing VMMs to advertise only host-supported
>> features to guests.
> 
> There needs to be a _lot_ more documentation explaining what these features are,
> and most importantly why it's safe/sane for KVM to advertise support to userspace
> without any corresponding code changes in KVM.
> 

Agreed. We don't have public documentation for most of these features at the
moment, but I will do my best to provide sufficient detail about each feature
and its safety implications in the next submission.

> The _EN flags in particular suggest some amount of emulation is required.

Right, I oversimplified this in the initial patch. I will investigate the _EN
bits more carefully and document what each one actually controls and whether KVM
needs to do anything beyond passthrough.

> 
> The patch also needs to be split up into related feature bundles (or invididual
> patches if each and every feature flag represents a completely independent feature).
> 

Makes sense. I will do thorough research on these features and group them into
logical bundles based on their functionality for the next version.

>> Link: https://lore.kernel.org/all/b3632083-f8ff-4127-a488-05a2c7acf1ad@xxxxxxxxxx/
>> Signed-off-by: Ewan Hai <ewanhai-oc@xxxxxxxxxxx>
>> ---
>>  arch/x86/kvm/cpuid.c         | 14 ++++++++++++++
>>  arch/x86/kvm/reverse_cpuid.h | 19 +++++++++++++++++++
>>  2 files changed, 33 insertions(+)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 88a5426674a1..529705079904 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -1242,8 +1242,12 @@ void kvm_set_cpu_caps(void)
>>               kvm_cpu_cap_set(X86_FEATURE_NULL_SEL_CLR_BASE);
>>
>>       kvm_cpu_cap_init(CPUID_C000_0001_EDX,
>> +             F(SM2),
>> +             F(SM2_EN),
>>               F(XSTORE),
>>               F(XSTORE_EN),
>> +             F(CCS),
>> +             F(CCS_EN),
>>               F(XCRYPT),
>>               F(XCRYPT_EN),
>>               F(ACE2),
>> @@ -1252,6 +1256,16 @@ void kvm_set_cpu_caps(void)
>>               F(PHE_EN),
>>               F(PMM),
>>               F(PMM_EN),
>> +             F(PARALLAX),
>> +             F(PARALLAX_EN),
>> +             F(TM3),
>> +             F(TM3_EN),
>> +             F(RNG2),
>> +             F(RNG2_EN),
>> +             F(PHE2),
>> +             F(PHE2_EN),
>> +             F(RSA),
>> +             F(RSA_EN),
>>       );
>>
>>       /*
>> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
>> index 81b4a7acf72e..33e6a2755c84 100644
>> --- a/arch/x86/kvm/reverse_cpuid.h
>> +++ b/arch/x86/kvm/reverse_cpuid.h
>> @@ -59,6 +59,25 @@
>>  #define KVM_X86_FEATURE_TSA_SQ_NO    KVM_X86_FEATURE(CPUID_8000_0021_ECX, 1)
>>  #define KVM_X86_FEATURE_TSA_L1_NO    KVM_X86_FEATURE(CPUID_8000_0021_ECX, 2)
>>
>> +/*
>> + * Zhaoxin/Centaur-defined CPUID level 0xC0000001 (EDX) features that are
>> + * currently KVM-only and not defined in cpufeatures.h.
>> + */
>> +#define X86_FEATURE_SM2             KVM_X86_FEATURE(CPUID_C000_0001_EDX, 0)
>> +#define X86_FEATURE_SM2_EN          KVM_X86_FEATURE(CPUID_C000_0001_EDX, 1)
>> +#define X86_FEATURE_CCS             KVM_X86_FEATURE(CPUID_C000_0001_EDX, 4)
>> +#define X86_FEATURE_CCS_EN          KVM_X86_FEATURE(CPUID_C000_0001_EDX, 5)
>> +#define X86_FEATURE_PARALLAX        KVM_X86_FEATURE(CPUID_C000_0001_EDX, 16)
>> +#define X86_FEATURE_PARALLAX_EN     KVM_X86_FEATURE(CPUID_C000_0001_EDX, 17)
>> +#define X86_FEATURE_TM3             KVM_X86_FEATURE(CPUID_C000_0001_EDX, 20)
>> +#define X86_FEATURE_TM3_EN          KVM_X86_FEATURE(CPUID_C000_0001_EDX, 21)
>> +#define X86_FEATURE_RNG2            KVM_X86_FEATURE(CPUID_C000_0001_EDX, 22)
>> +#define X86_FEATURE_RNG2_EN         KVM_X86_FEATURE(CPUID_C000_0001_EDX, 23)
>> +#define X86_FEATURE_PHE2            KVM_X86_FEATURE(CPUID_C000_0001_EDX, 25)
>> +#define X86_FEATURE_PHE2_EN         KVM_X86_FEATURE(CPUID_C000_0001_EDX, 26)
>> +#define X86_FEATURE_RSA             KVM_X86_FEATURE(CPUID_C000_0001_EDX, 27)
>> +#define X86_FEATURE_RSA_EN          KVM_X86_FEATURE(CPUID_C000_0001_EDX, 28)
>> +
>>  struct cpuid_reg {
>>       u32 function;
>>       u32 index;
>> --
>> 2.34.1
>>