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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 23 Apr 2021 09:31:07 -0700
Message-ID: <CALMp9eTTC0kFbRt8TBC3x8N4mDism-xg3xEFSCPGiGc20pZ3ZQ@mail.gmail.com>
Subject: Re: [PATCH v2 1/4] KVM: x86: invert KVM_HYPERCALL to default to VMMCALL
To:     Ashish Kalra <Ashish.Kalra@amd.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Joerg Roedel <joro@8bytes.org>,
        Borislav Petkov <bp@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Steve Rutherford <srutherford@google.com>,
        Sean Christopherson <seanjc@google.com>,
        venu.busireddy@oracle.com, Brijesh Singh <brijesh.singh@amd.com>
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On Fri, Apr 23, 2021 at 9:00 AM Ashish Kalra <Ashish.Kalra@amd.com> wrote:
>
> From: Ashish Kalra <ashish.kalra@amd.com>
>
> KVM hypercall framework relies on alternative framework to patch the
> VMCALL -> VMMCALL on AMD platform. If a hypercall is made before
> apply_alternative() is called then it defaults to VMCALL. The approach
> works fine on non SEV guest. A VMCALL would causes #UD, and hypervisor
> will be able to decode the instruction and do the right things. But
> when SEV is active, guest memory is encrypted with guest key and
> hypervisor will not be able to decode the instruction bytes.
>
> So invert KVM_HYPERCALL and X86_FEATURE_VMMCALL to default to VMMCALL
> and opt into VMCALL.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: x86@kernel.org
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>  arch/x86/include/asm/kvm_para.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> index 338119852512..fda2fe0d1b10 100644
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -19,7 +19,7 @@ static inline bool kvm_check_and_clear_guest_paused(void)
>  #endif /* CONFIG_KVM_GUEST */
>
>  #define KVM_HYPERCALL \
> -        ALTERNATIVE("vmcall", "vmmcall", X86_FEATURE_VMMCALL)
> +       ALTERNATIVE("vmmcall", "vmcall", X86_FEATURE_VMCALL)
>
>  /* For KVM hypercalls, a three-byte sequence of either the vmcall or the vmmcall
>   * instruction.  The hypervisor may replace it with something else but only the
> --
> 2.17.1
>

Won't this result in the same problem when Intel implements full VM encryption?