vmm(4): Check for and allow empty GHCB; only clear valid bitmap

The GHCB valid bitmap indicates wether the GHCB contains a request
or not.  When no bits are set, ignore the GHCB and do not sync with
vCPU state.

To clear/invalidate the GHCB just zero out the valid bitmap instead
of the full GHCB.

ok mlarkin@

sys/arch/amd64/amd64/ghcb.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ghcb.c,v 1.7 2026/01/15 12:11:51 hshoexer Exp $	*/
+/*	$OpenBSD: ghcb.c,v 1.8 2026/02/16 15:06:03 hshoexer Exp $	*/
 
 /*
  * Copyright (c) 2024, 2025 Hans-Joerg Hoexer <hshoexer@genua.de>
@@ -53,13 +53,29 @@ paddr_t ghcb_paddr;
 /*
  * ghcb_clear
  *
- * Clear GHCB by setting to all 0.
+ * Clear GHCB valid bitmap to all 0.  After that, the GHCB does
+ * not contain a valid request.
  * Used by host and guest.
  */
 void
 ghcb_clear(struct ghcb_sa *ghcb)
 {
-	memset(ghcb, 0, sizeof(*ghcb));
+	memset(&ghcb->valid_bitmap, 0, sizeof(ghcb->valid_bitmap));
+}
+
+/*
+ * ghcb_empty
+ *
+ * If the GHCB is all clear, it contains no requests.
+ * Used by host only.
+ */
+int
+ghcb_empty(struct ghcb_sa *ghcb)
+{
+	static const uint8_t bm_allzero[GHCB_VB_SZ];
+
+	return (memcmp(&bm_allzero, &ghcb->valid_bitmap,
+           sizeof(bm_allzero)) == 0);
 }
 
 /*

sys/arch/amd64/amd64/vmm_machdep.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: vmm_machdep.c,v 1.70 2026/02/16 12:43:58 hshoexer Exp $ */
+/* $OpenBSD: vmm_machdep.c,v 1.71 2026/02/16 15:06:03 hshoexer Exp $ */
 /*
  * Copyright (c) 2014 Mike Larkin <mlarkin@openbsd.org>
  *
@@ -4347,6 +4347,8 @@ svm_vmgexit_sync_host(struct vcpu *vcpu)
 		return (0);
 
 	ghcb = (struct ghcb_sa *)vcpu->vc_svm_ghcb_va;
+	if (ghcb_empty(ghcb))
+		return (0);
 	if (!ghcb_valid(ghcb))
 		return (EINVAL);
 	valid_bm = ghcb->valid_bitmap;

sys/arch/amd64/include/ghcb.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ghcb.h,v 1.6 2025/09/17 18:37:44 sf Exp $	*/
+/*	$OpenBSD: ghcb.h,v 1.7 2026/02/16 15:06:03 hshoexer Exp $	*/
 
 /*
  * Copyright (c) 2024, 2025 Hans-Joerg Hoexer <hshoexer@genua.de>
@@ -128,6 +128,7 @@ int	ghcb_valbm_set(uint8_t *, int);
 int	ghcb_valbm_isset(uint8_t *, int);
 int	ghcb_verify_bm(uint8_t *, uint8_t *);
 int	ghcb_valid(struct ghcb_sa *);
+int	ghcb_empty(struct ghcb_sa *);
 
 void	ghcb_sync_val(int, int, struct ghcb_sync *);
 void	ghcb_sync_out(struct trapframe *, const struct ghcb_extra_regs *,