Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Sep 23, 2020 at 03:27:39PM -0500, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> The INVD instruction intercept performs emulation. Emulation can't be done
> on an SEV guest because the guest memory is encrypted.
> 
> Provide a dedicated intercept routine for the INVD intercept. Within this
> intercept routine just skip the instruction for an SEV guest, since it is
> emulated as a NOP anyway.
> 
> Fixes: 1654efcbc431 ("KVM: SVM: Add KVM_SEV_INIT command")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kvm/svm/svm.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c91acabf18d0..332ec4425d89 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2183,6 +2183,17 @@ static int iret_interception(struct vcpu_svm *svm)
>  	return 1;
>  }
>  
> +static int invd_interception(struct vcpu_svm *svm)
> +{
> +	/*
> +	 * Can't do emulation on an SEV guest and INVD is emulated
> +	 * as a NOP, so just skip the instruction.
> +	 */
> +	return (sev_guest(svm->vcpu.kvm))
> +		? kvm_skip_emulated_instruction(&svm->vcpu)
> +		: kvm_emulate_instruction(&svm->vcpu, 0);

Is there any reason not to do kvm_skip_emulated_instruction() for both SEV
and legacy?  VMX has the same odd kvm_emulate_instruction() call, but AFAICT
that's completely unecessary, i.e. VMX can also convert to a straight skip.

> +}
> +
>  static int invlpg_interception(struct vcpu_svm *svm)
>  {
>  	if (!static_cpu_has(X86_FEATURE_DECODEASSISTS))
> @@ -2774,7 +2785,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
>  	[SVM_EXIT_RDPMC]			= rdpmc_interception,
>  	[SVM_EXIT_CPUID]			= cpuid_interception,
>  	[SVM_EXIT_IRET]                         = iret_interception,
> -	[SVM_EXIT_INVD]                         = emulate_on_interception,
> +	[SVM_EXIT_INVD]                         = invd_interception,
>  	[SVM_EXIT_PAUSE]			= pause_interception,
>  	[SVM_EXIT_HLT]				= halt_interception,
>  	[SVM_EXIT_INVLPG]			= invlpg_interception,
> -- 
> 2.28.0
>