Index: usr.sbin/bhyve/block_if.h
===================================================================
--- usr.sbin/bhyve/block_if.h
+++ usr.sbin/bhyve/block_if.h
@@ -41,7 +41,7 @@
 #include <sys/uio.h>
 #include <sys/unistd.h>

-#define BLOCKIF_IOV_MAX                33      /* not practical to be IOV_MAX */
+#define BLOCKIF_IOV_MAX                128     /* not practical to be IOV_MAX */

 struct blockif_req {
        struct iovec    br_iov[BLOCKIF_IOV_MAX];
Index: usr.sbin/bhyve/block_if.c
===================================================================
--- usr.sbin/bhyve/block_if.c
+++ usr.sbin/bhyve/block_if.c
@@ -62,7 +62,7 @@
 #define BLOCKIF_SIG    0xb109b109

 #define BLOCKIF_NUMTHR 8
-#define BLOCKIF_MAXREQ (64 + BLOCKIF_NUMTHR)
+#define BLOCKIF_MAXREQ (128 + BLOCKIF_NUMTHR)

 enum blockop {
        BOP_READ,
Index: usr.sbin/bhyve/pci_virtio_block.c
===================================================================
--- usr.sbin/bhyve/pci_virtio_block.c
+++ usr.sbin/bhyve/pci_virtio_block.c
@@ -55,7 +55,7 @@
 #include "virtio.h"
 #include "block_if.h"

-#define VTBLK_RINGSZ   64
+#define VTBLK_RINGSZ   128

 #define VTBLK_S_OK     0
 #define VTBLK_S_IOERR  1
@@ -350,7 +350,14 @@
        /* setup virtio block config space */
        sc->vbsc_cfg.vbc_capacity = size / DEV_BSIZE; /* 512-byte units */
        sc->vbsc_cfg.vbc_size_max = 0;  /* not negotiated */
-       sc->vbsc_cfg.vbc_seg_max = BLOCKIF_IOV_MAX;
+       /*
+        * If Linux is presented with a seg_max greater than the virtio queue
+        * size, it can stumble into situations where it violates its own
+        * invariants and panics.  For safety, we keep seg_max clamped, paying
+        * heed to the two extra descriptors needed for the header and status
+        * of a request.
+        */
+       sc->vbsc_cfg.vbc_seg_max = MIN(VTBLK_RINGSZ - 2, BLOCKIF_IOV_MAX);
        sc->vbsc_cfg.vbc_geometry.cylinders = 0;        /* no geometry */
        sc->vbsc_cfg.vbc_geometry.heads = 0;
        sc->vbsc_cfg.vbc_geometry.sectors = 0;