linux-kernel - Re: [RFC PATCH v6 090/104] KVM: TDX: Handle TDX PV CPUID hypercall

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Message-ID: <20220629101356.GA882746@ls.amr.corp.intel.com>
Date:   Wed, 29 Jun 2022 03:13:56 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sagi Shahar <sagis@...gle.com>
Cc:     "Yamahata, Isaku" <isaku.yamahata@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v6 090/104] KVM: TDX: Handle TDX PV CPUID hypercall

On Tue, Jun 14, 2022 at 11:15:00AM -0700,
Sagi Shahar <sagis@...gle.com> wrote:

> On Thu, May 5, 2022 at 11:16 AM <isaku.yamahata@...el.com> wrote:
> >
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> > Wire up TDX PV CPUID hypercall to the KVM backend function.
> >
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > ---
> >  arch/x86/kvm/vmx/tdx.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 9c712f661a7c..c7cdfee397ec 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -946,12 +946,34 @@ static int tdx_emulate_vmcall(struct kvm_vcpu *vcpu)
> >         return 1;
> >  }
> >
> > +static int tdx_emulate_cpuid(struct kvm_vcpu *vcpu)
> > +{
> > +       u32 eax, ebx, ecx, edx;
> > +
> > +       /* EAX and ECX for cpuid is stored in R12 and R13. */
> > +       eax = tdvmcall_a0_read(vcpu);
> > +       ecx = tdvmcall_a1_read(vcpu);
> > +
> > +       kvm_cpuid(vcpu, &eax, &ebx, &ecx, &edx, true);
> 
> According to the GHCI spec section 3.6
> (TDG.VP.VMCALL<Instruction.CPUID>) we should return
> VMCALL_INVALID_OPERAND if an invalid CPUID is requested.
> 
> kvm_cpuid already returns false in this case so we should use that
> return value to set the tdvmcall return code in case of invalid leaf.

Based on CPUID instruction, cpuid results in #UD when lock prefix is used or
earlier CPU that doesn't support cpuid instruction.
So I'm not sure what CPUID input result in INVALID_OPERAND error.
Does the following make sense for you?

--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1347,7 +1347,7 @@ static int tdx_emulate_cpuid(struct kvm_vcpu *vcpu)
        eax = tdvmcall_a0_read(vcpu);
        ecx = tdvmcall_a1_read(vcpu);

-       kvm_cpuid(vcpu, &eax, &ebx, &ecx, &edx, true);
+       kvm_cpuid(vcpu, &eax, &ebx, &ecx, &edx, false);

        tdvmcall_a0_write(vcpu, eax);
        tdvmcall_a1_write(vcpu, ebx);

thanks,

-- 
Isaku Yamahata <isaku.yamahata@...il.com>