Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported

From: Paolo Bonzini <pbonzini@redhat.com>
To: Reiji Watanabe <reijiw@google.com>,
	Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Xiaoyao Li <xiaoyao.li@intel.com>
Subject: Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported
Date: Wed, 5 May 2021 10:01:54 +0200	[thread overview]
Message-ID: <92131802-68b7-3d27-3e61-9b388bfcbc7f@redhat.com> (raw)
In-Reply-To: <CAAeT=FycnR2BonmiHSWobsLCGTuQuTS3kg9x_eYCKLRQGOvYzQ@mail.gmail.com>

On 05/05/21 05:51, Reiji Watanabe wrote:
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -637,7 +637,8 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>>          case 7:
>>                  entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>>                  entry->eax = 0;
>> -               entry->ecx = F(RDPID);
>> +               if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
>> +                       entry->ecx = F(RDPID);
>>                  ++array->nent;
>>          default:
>>                  break;
> I'm wondering if entry->ecx should be set to F(RDPID) here
> even if the CPU supports RDPID natively.
> (i.e. kvm_cpu_cap_has(X86_FEATURE_RDPID) is true)
> 
> The document "Documentation/virt/kvm/api.rst" says:
> ---
> 4.88 KVM_GET_EMULATED_CPUID
> ---------------------------
> <...>
> Userspace can use the information returned by this ioctl to query
> which features are emulated by kvm instead of being present natively.
> ---

Setting it always is consistent with the treatment of MOVBE above. 
Either way is okay but it should be done for both bits.

Paolo