[PATCH RFC v7 60/64] KVM: SVM: Sync the GHCB scratch buffer using already mapped ghcb

[Michael Roth]

From: Ashish Kalra <ashish.kalra@xxxxxxx>

Using kvm_write_guest() to sync the GHCB scratch buffer can fail
due to host mapping being 2M, but RMP being 4K. The page fault handling
in do_user_addr_fault() fails to split the 2M page to handle RMP fault due
to it being called here in a non-preemptible context. Instead use
the already kernel mapped ghcb to sync the scratch buffer when the
scratch buffer is contained within the GHCB.

Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
---
 arch/x86/kvm/svm/sev.c | 29 +++++++++++++++++++++--------
 arch/x86/kvm/svm/svm.h |  2 ++
 2 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b0f25ced7bcf..4de952d1d446 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3036,6 +3036,24 @@ static bool sev_es_sync_to_ghcb(struct vcpu_svm *svm)
 	ghcb_set_sw_exit_info_1(ghcb, svm->sev_es.ghcb_sw_exit_info_1);
 	ghcb_set_sw_exit_info_2(ghcb, svm->sev_es.ghcb_sw_exit_info_2);
 
+	/* Sync the scratch buffer area. */
+	if (svm->sev_es.ghcb_sa_sync) {
+		if (svm->sev_es.ghcb_sa_contained) {
+			memcpy(ghcb->shared_buffer + svm->sev_es.ghcb_sa_offset,
+			       svm->sev_es.ghcb_sa, svm->sev_es.ghcb_sa_len);
+		} else {
+			int ret;
+
+			ret = kvm_write_guest(svm->vcpu.kvm,
+					      svm->sev_es.ghcb_sa_gpa,
+					      svm->sev_es.ghcb_sa, svm->sev_es.ghcb_sa_len);
+			if (ret)
+				pr_warn_ratelimited("unmap_ghcb: kvm_write_guest failed while syncing scratch area, gpa: %llx, ret: %d\n",
+						    svm->sev_es.ghcb_sa_gpa, ret);
+		}
+		svm->sev_es.ghcb_sa_sync = false;
+	}
+
 	trace_kvm_vmgexit_exit(svm->vcpu.vcpu_id, ghcb);
 
 	svm_unmap_ghcb(svm, &map);
@@ -3248,14 +3266,6 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm)
 	if (!svm->sev_es.ghcb_in_use)
 		return;
 
-	 /* Sync the scratch buffer area. */
-	if (svm->sev_es.ghcb_sa_sync) {
-		kvm_write_guest(svm->vcpu.kvm,
-				svm->sev_es.ghcb_sa_gpa,
-				svm->sev_es.ghcb_sa, svm->sev_es.ghcb_sa_len);
-		svm->sev_es.ghcb_sa_sync = false;
-	}
-
 	sev_es_sync_to_ghcb(svm);
 
 	svm->sev_es.ghcb_in_use = false;
@@ -3321,6 +3331,8 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 			       scratch_gpa_beg, scratch_gpa_end);
 			goto e_scratch;
 		}
+		svm->sev_es.ghcb_sa_contained = true;
+		svm->sev_es.ghcb_sa_offset = scratch_gpa_beg - ghcb_scratch_beg;
 	} else {
 		/*
 		 * The guest memory must be read into a kernel buffer, so
@@ -3331,6 +3343,7 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 			       len, GHCB_SCRATCH_AREA_LIMIT);
 			goto e_scratch;
 		}
+		svm->sev_es.ghcb_sa_contained = false;
 	}
 
 	if (svm->sev_es.ghcb_sa_alloc_len < len) {
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 4692ada13f02..38aa579f6f70 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -217,6 +217,8 @@ struct vcpu_sev_es_state {
 	u64 ghcb_sa_gpa;
 	u32 ghcb_sa_alloc_len;
 	bool ghcb_sa_sync;
+	bool ghcb_sa_contained;
+	u32 ghcb_sa_offset;
 
 	/*
 	 * SEV-ES support to hold the sw_exit_info return values to be
-- 
2.25.1