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

[Sean Christopherson <seanjc@google.com>]

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.

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 {
-- 
2.30.0.280.ga3ce27912f-goog