[Qemu-devel] [RFC v9 05/27] virtio-blk: Do cheapest possible memory mapping

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

Instead of using QEMU memory access functions, grab the host address of
guest physical address zero and simply add to this base address.

This not only simplifies vring mapping but will also make virtqueue
element access cheap by avoiding QEMU memory access functions in the I/O
code path.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 hw/virtio-blk.c |   58 ++++++++++++++++++++++++++++---------------------------
 1 file changed, 30 insertions(+), 28 deletions(-)

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 4c790a3..abd9386 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -51,6 +51,8 @@ typedef struct VirtIOBlock
     EventNotifier io_notifier;      /* Linux AIO eventfd */
     EventHandler io_handler;        /* Linux AIO completion handler */
     EventHandler notify_handler;    /* virtqueue notify handler */
+
+    void *phys_mem_zero_host_ptr;   /* host pointer to guest RAM */
 } VirtIOBlock;
 
 static VirtIOBlock *to_virtio_blk(VirtIODevice *vdev)
@@ -58,43 +60,44 @@ static VirtIOBlock *to_virtio_blk(VirtIODevice *vdev)
     return (VirtIOBlock *)vdev;
 }
 
+/* Map target physical address to host address
+ */
+static inline void *phys_to_host(VirtIOBlock *s, target_phys_addr_t phys)
+{
+    return s->phys_mem_zero_host_ptr + phys;
+}
+
+/* Setup for cheap target physical to host address conversion
+ *
+ * This is a hack for direct access to guest memory, we're not really allowed
+ * to do this.
+ */
+static void setup_phys_to_host(VirtIOBlock *s)
+{
+    target_phys_addr_t len = 4096; /* RAM is really much larger but we cheat */
+    s->phys_mem_zero_host_ptr = cpu_physical_memory_map(0, &len, 0);
+    if (!s->phys_mem_zero_host_ptr) {
+        fprintf(stderr, "setup_phys_to_host failed\n");
+        exit(1);
+    }
+}
+
 /* Map the guest's vring to host memory
  *
  * This is not allowed but we know the ring won't move.
  */
-static void map_vring(struct vring *vring, VirtIODevice *vdev, int n)
+static void map_vring(struct vring *vring, VirtIOBlock *s, VirtIODevice *vdev, int n)
 {
-    target_phys_addr_t physaddr, len;
-
     vring->num = virtio_queue_get_num(vdev, n);
-
-    physaddr = virtio_queue_get_desc_addr(vdev, n);
-    len = virtio_queue_get_desc_size(vdev, n);
-    vring->desc = cpu_physical_memory_map(physaddr, &len, 0);
-
-    physaddr = virtio_queue_get_avail_addr(vdev, n);
-    len = virtio_queue_get_avail_size(vdev, n);
-    vring->avail = cpu_physical_memory_map(physaddr, &len, 0);
-
-    physaddr = virtio_queue_get_used_addr(vdev, n);
-    len = virtio_queue_get_used_size(vdev, n);
-    vring->used = cpu_physical_memory_map(physaddr, &len, 0);
-
-    if (!vring->desc || !vring->avail || !vring->used) {
-        fprintf(stderr, "virtio-blk failed to map vring\n");
-        exit(1);
-    }
+    vring->desc = phys_to_host(s, virtio_queue_get_desc_addr(vdev, n));
+    vring->avail = phys_to_host(s, virtio_queue_get_avail_addr(vdev, n));
+    vring->used = phys_to_host(s, virtio_queue_get_used_addr(vdev, n));
 
     fprintf(stderr, "virtio-blk vring physical=%#lx desc=%p avail=%p used=%p\n",
             virtio_queue_get_ring_addr(vdev, n),
             vring->desc, vring->avail, vring->used);
 }
 
-static void unmap_vring(struct vring *vring, VirtIODevice *vdev, int n)
-{
-    cpu_physical_memory_unmap(vring->desc, virtio_queue_get_ring_size(vdev, n), 0, 0);
-}
-
 static void handle_io(void)
 {
     fprintf(stderr, "io completion happened\n");
@@ -149,7 +152,8 @@ static void add_event_handler(int epoll_fd, EventHandler *event_handler)
 
 static void data_plane_start(VirtIOBlock *s)
 {
-    map_vring(&s->vring, &s->vdev, 0);
+    setup_phys_to_host(s);
+    map_vring(&s->vring, s, &s->vdev, 0);
 
     /* Create epoll file descriptor */
     s->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
@@ -199,8 +203,6 @@ static void data_plane_stop(VirtIOBlock *s)
     s->vdev.binding->set_host_notifier(s->vdev.binding_opaque, 0, false);
 
     close(s->epoll_fd);
-
-    unmap_vring(&s->vring, &s->vdev, 0);
 }
 
 static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t val)
-- 
1.7.10.4