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

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Date:   Mon, 18 Jul 2022 15:37:35 -0700
From:   Sagi Shahar <sagis@...gle.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     "Yamahata, Isaku" <isaku.yamahata@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, 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 Wed, Jun 29, 2022 at 3:14 AM Isaku Yamahata <isaku.yamahata@...il.com> wrote:
>
> 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,

If any CPUID request is considered valid, then perhaps the spec itself
needs to be updated.

Right now it clearly states that TDG.VP.VMCALL_INVALID_OPERAND is
returned if "Invalid CPUID requested" which I understood as a
non-existing leaf. But if you say that a non-existing leaf is still a
valid CPUID request than I'm not sure what "Invalid CPUID requested"
means in the spec itself.

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