Re: [PATCH v2 1/4] KVM: x86: invert KVM_HYPERCALL to default to VMMCALL

[Jim Mattson]

On Fri, Apr 23, 2021 at 9:00 AM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
>
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Joerg Roedel <joro@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>  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?