vhost: Add USED_EVENT and AVAIL_EVENT macros

Open-coding this was kind of awful. I mean, it's *still* fairly awful
but now we can hide it away in the macro and never think about it again.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

diff --git a/vhost.c b/vhost.c
index f5414496c0b3ef4a87c25b485e848db212664146..a311d9ff2920334d15cb2def99d71ecb56862656 100644 (file)
--- a/vhost.c
+++ b/vhost.c
@@ -291,6 +291,43 @@ void shutdown_vhost(struct openconnect_info *vpninfo)
        free_vring(vpninfo, &vpninfo->tx_vring);
 }
 
+/* used_event is the uint16_t element after the end of the
+ * avail ring:
+ *
+ *     struct virtq_avail {
+ *             le16 flags;
+ *             le16 idx;
+ *             le16 ring[ Queue Size ];
+ *             le16 used_event;
+ *     };
+ */
+
+#define USED_EVENT(v, r) ((r)->avail->ring[(v)->vhost_ring_size])
+
+/* avail_event is the uint16_t element after the end of the
+ * used ring, which is slightly less trivial to reference
+ * than the used_event:
+ *
+ *     struct virtq_used_elem {
+ *             le32 id;
+ *             le32 len;
+ *     };
+ *
+ *     struct virtq_used {
+ *             le16 flags;
+ *             le16 idx;
+ *             struct virtq_used_elem ring[ Queue Size ];
+ *             le16 avail_event;
+ *     };
+ *
+ * So if we thought of it as an array of 16-bit values, 'flags' would
+ * be at element [0], 'idx' at [1], the ring would start at [2], the
+ * *second* element of the ring would be at [ 2 + 4 ] since each element
+ * is as big as four 16-bit values, and thus avail_event would be at
+ *  [2 + 4 * RING_SIZE ]
+ */
+#define AVAIL_EVENT(v, r) ((&(r)->used->flags)[2 + ((v)->vhost_ring_size * 4)])
+
 static void dump_vring(struct openconnect_info *vpninfo, struct oc_vring *ring)
 {
        vpn_progress(vpninfo, PRG_ERR,
@@ -300,12 +337,13 @@ static void dump_vring(struct openconnect_info *vpninfo, struct oc_vring *ring)
 
        vpn_progress(vpninfo, PRG_ERR, "#   ADDR         AVAIL         USED\n");
 
+       /* Not an off-by-one; it's dumping avail_event and used_event too. */
        for (int i = 0; i < vpninfo->vhost_ring_size + 1; i++)
                vpn_progress(vpninfo, PRG_ERR,
                             "%d %p %x %x\n", i,
                             (void *)vio64(ring->desc[i].addr),
                             vio16(ring->avail->ring[i]),
-                            vio16(ring->used->ring[i].id));
+                            vio32(ring->used->ring[i].id));
 }
 
 /* With thanks to Eugenio Pérez Martin <eperezma@redhat.com> for writing
@@ -399,7 +437,7 @@ static inline int process_ring(struct openconnect_info *vpninfo, int tx, uint64_
                         * our CPU on the RX and crypto; there's not a lot of point
                         * otherwise. */
                        if (!*kick && vpninfo->incoming_queue.count < vpninfo->max_qlen / 2 &&
-                           next_avail == (&ring->used->flags)[2 + (vpninfo->vhost_ring_size * 4)]) {
+                           next_avail == AVAIL_EVENT(vpninfo, ring)) {
                                if (!os_write_tun(vpninfo, this)) {
                                        vpninfo->stats.rx_pkts++;
                                        vpninfo->stats.rx_bytes += this->len;
@@ -442,7 +480,7 @@ static inline int process_ring(struct openconnect_info *vpninfo, int tx, uint64_
 
                ring->avail->idx = vio16(++next_avail);
                barrier();
-               uint16_t avail_event = (&ring->used->flags)[2 + (vpninfo->vhost_ring_size * 4)];
+               uint16_t avail_event = AVAIL_EVENT(vpninfo, ring);
                barrier();
                if (avail_event == vio16(next_avail-1))
                        *kick = 1;
@@ -462,9 +500,8 @@ static int set_ring_wake(struct openconnect_info *vpninfo, int tx)
        struct oc_vring *ring = tx ? &vpninfo->tx_vring : &vpninfo->rx_vring;
        uint16_t wake_idx = vio16(ring->seen_used);
 
-       /* Ask it to wake us if the used idx moves on. Note: used_event
-        * is at the end of the *avail* ring, and vice versa. */
-       ring->avail->ring[vpninfo->vhost_ring_size] = wake_idx;
+       /* Ask it to wake us if the used idx moves on. */
+       USED_EVENT(vpninfo, ring) = wake_idx;
        barrier();
 
        /* If it already did, loop again immediately */