Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210504171734.1434054-1-seanjc@google.com> <20210504171734.1434054-3-seanjc@google.com>
In-Reply-To: <20210504171734.1434054-3-seanjc@google.com>
From:   Reiji Watanabe <reijiw@google.com>
Date:   Tue, 4 May 2021 20:51:41 -0700
Message-ID: <CAAeT=FycnR2BonmiHSWobsLCGTuQuTS3kg9x_eYCKLRQGOvYzQ@mail.gmail.com>
Subject: Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

> --- 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