Re: [PATCH 3/3] KVM: SVM: Sync GPRs to the GHCB only after VMGEXIT

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/22/21 5:50 PM, Sean Christopherson wrote:
> Sync GPRs to the GHCB on VMRUN only if a sync is needed, i.e. if the
> previous exit was a VMGEXIT and the guest is expecting some data back.
> 

The start of sev_es_sync_to_ghcb() checks if the GHCB has been mapped, 
which only occurs on VMGEXIT, and exits early if not. And 
sev_es_sync_from_ghcb() is only called if the GHCB has been successfully 
mapped. The only thing in between is sev_es_validate_vmgexit(), which will 
terminate the VM on error. So I don't think this patch is needed.

Thanks,
Tom

> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/svm/sev.c | 15 ++++++++++-----
>   arch/x86/kvm/svm/svm.h |  1 +
>   2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index ac652bc476ae..9bd1e1650eb3 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1418,10 +1418,13 @@ static void sev_es_sync_to_ghcb(struct vcpu_svm *svm)
>   	 * Copy their values, even if they may not have been written during the
>   	 * VM-Exit.  It's the guest's responsibility to not consume random data.
>   	 */
> -	ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
> -	ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
> -	ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
> -	ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
> +	if (svm->need_sync_to_ghcb) {
> +		ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
> +		ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
> +		ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
> +		ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
> +		svm->need_sync_to_ghcb = false;
> +	}
>   }
>   
>   static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
> @@ -1441,8 +1444,10 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
>   	 * VMMCALL allows the guest to provide extra registers. KVM also
>   	 * expects RSI for hypercalls, so include that, too.
>   	 *
> -	 * Copy their values to the appropriate location if supplied.
> +	 * Copy their values to the appropriate location if supplied, and
> +	 * flag that a sync back to the GHCB is needed on the next VMRUN.
>   	 */
> +	svm->need_sync_to_ghcb = true;
>   	memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));
>   
>   	vcpu->arch.regs[VCPU_REGS_RAX] = ghcb_get_rax_if_valid(ghcb);
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 0fe874ae5498..4e2e5f9fbfc2 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -192,6 +192,7 @@ struct vcpu_svm {
>   	u64 ghcb_sa_len;
>   	bool ghcb_sa_sync;
>   	bool ghcb_sa_free;
> +	bool need_sync_to_ghcb;
>   };
>   
>   struct svm_cpu_data {
>