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

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Date:   Tue, 4 May 2021 20:51:41 -0700
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported

> --- 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.
---

Thanks,
Reiji