bhyve: allow reading of fwctl signature multiple times

At the moment, you only have one single chance to read the fwctl
signature. At boot bhyve is in the state IDENT_WAIT. It's then
possible to switch to IDENT_SEND. After bhyve sends the signature,
it switches to REQ. From now on it's impossible to switch back to
IDENT_SEND to read the signature. For that reason, only a single
driver can read the signature. A guest can't use two drivers to
identify that fwctl is present. It gets even worse when using
OVMF. OVMF uses a library to access fwctl. Therefore, every single
OVMF driver would try to read the signature. Currently, only a
single OVMF driver accesses the fwctl. So, there's no issue with
it yet. However, no OS driver would have a chance to detect fwctl when
using OVMF because it's signature was already consumed by OVMF.

Reviewed by:    markj
MFC after:      2 weeks
Sponsored by:   Beckhoff Automation GmbH & Co. KG
Differential Revision:  https://reviews.freebsd.org/D31981

1 files changed, 12 insertions, 10 deletions

diff --git a/usr.sbin/bhyve/fwctl.c b/usr.sbin/bhyve/fwctl.c
index a91f3d14fb78..8faae30989e6 100644
--- a/usr.sbin/bhyve/fwctl.c
+++ b/usr.sbin/bhyve/fwctl.c
@@ -472,16 +472,18 @@ fwctl_inb(void)
 static void
 fwctl_outw(uint16_t val)
 {
-	switch (be_state) {
-	case IDENT_WAIT:
-		if (val == 0) {
-			be_state = IDENT_SEND;
-			ident_idx = 0;
-		}
-		break;
-	default:
-		/* ignore */
-		break;
+	if (be_state == DORMANT) {
+		return;
+	}
+
+	if (val == 0) {
+		/*
+		 * The guest wants to read the signature. It's possible that the
+		 * guest is unaware of the fwctl state at this moment. For that
+		 * reason, reset the state machine unconditionally.
+		 */
+		be_state = IDENT_SEND;
+		ident_idx = 0;
 	}
 }