Index: usr.sbin/bhyve/pci_ahci.c
===================================================================
--- usr.sbin/bhyve/pci_ahci.c
+++ usr.sbin/bhyve/pci_ahci.c
@@ -2093,7 +2093,7 @@
 	struct pci_ahci_softc *sc = pi->pi_arg;
 
 	assert(baridx == 5);
-	assert(size == 4);
+	assert((offset % 4) == 0 && size == 4);
 
 	pthread_mutex_lock(&sc->mtx);
 
@@ -2182,24 +2182,29 @@
 
 static uint64_t
 pci_ahci_read(struct vmctx *ctx, int vcpu, struct pci_devinst *pi, int baridx,
-    uint64_t offset, int size)
+    uint64_t regoff, int size)
 {
 	struct pci_ahci_softc *sc = pi->pi_arg;
+	uint64_t offset;
 	uint32_t value;
 
 	assert(baridx == 5);
-	assert(size == 4);
+	assert(size == 1 || size == 2 || size == 4);
+	assert((regoff & (size - 1)) == 0);
 
 	pthread_mutex_lock(&sc->mtx);
 
+	offset = regoff & ~0x3;	    /* round down to a multiple of 4 bytes */
 	if (offset < AHCI_OFFSET)
 		value = pci_ahci_host_read(sc, offset);
 	else if (offset < AHCI_OFFSET + sc->ports * AHCI_STEP)
 		value = pci_ahci_port_read(sc, offset);
 	else {
 		value = 0;
-		WPRINTF("pci_ahci: unknown i/o read offset 0x%"PRIx64"\n", offset);
+		WPRINTF("pci_ahci: unknown i/o read offset 0x%"PRIx64"\n",
+		    regoff);
 	}
+	value >>= 8 * (regoff & 0x3);
 
 	pthread_mutex_unlock(&sc->mtx);