linux-kernel - Re: [PATCH 2/4] KVM: VMX: Unconditionally clear CPUID.INVPCID if !CPUID.PCID

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Date:   Wed, 23 Sep 2020 10:15:11 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] KVM: VMX: Unconditionally clear CPUID.INVPCID if !CPUID.PCID

On Wed, Sep 23, 2020 at 9:51 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> If PCID is not exposed to the guest, clear INVPCID in the guest's CPUID
> even if the VMCS INVPCID enable is not supported.  This will allow
> consolidating the secondary execution control adjustment code without
> having to special case INVPCID.
>
> Technically, this fixes a bug where !CPUID.PCID && CPUID.INVCPID would
> result in unexpected guest behavior (#UD instead of #GP/#PF), but KVM
> doesn't support exposing INVPCID if it's not supported in the VMCS, i.e.
> such a config is broken/bogus no matter what.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index cfed29329e4f..57e48c5a1e91 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4149,16 +4149,22 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
>                 }
>         }
>
> +       /*
> +        * Expose INVPCID if and only if PCID is also exposed to the guest.
> +        * INVPCID takes a #UD when it's disabled in the VMCS, but a #GP or #PF
> +        * if CR4.PCIDE=0.  Enumerating CPUID.INVPCID=1 would lead to incorrect
> +        * behavior from the guest perspective (it would expect #GP or #PF).
> +        */
> +       if (!guest_cpuid_has(vcpu, X86_FEATURE_PCID))
> +               guest_cpuid_clear(vcpu, X86_FEATURE_INVPCID);
> +

I thought the general rule for userspace provided CPUID bits was that
kvm only made any adjustments necessary to prevent bad things from
happening at the host level. Proper guest semantics are entirely the
responsibility of userspace. Or did I misunderstand?