kvm tools: Improve virtio blk request processing

There are at most bdev->reqs[VIRTIO_BLK_QUEUE_SIZE] outstanding requests
at any time.  We can simply use the head of each request to fetch the
right 'struct blk_dev_req' in bdev->reqs[].

So, we can eliminate the list and lock operations which introduced by
virtio_blk_req_{pop, push}.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/kvm/virtio.h

@@ -58,7 +58,8 @@ static inline void *guest_pfn_to_host(struct kvm *kvm, u32 pfn)
 struct vring_used_elem *virt_queue__set_used_elem(struct virt_queue *queue, u32 head, u32 len);
 
 bool virtio_queue__should_signal(struct virt_queue *vq);
-u16 virt_queue__get_iov(struct virt_queue *queue, struct iovec iov[], u16 *out, u16 *in, struct kvm *kvm);
+u16 virt_queue__get_iov(struct virt_queue *vq, struct iovec iov[], u16 *out, u16 *in, struct kvm *kvm);
+u16 virt_queue__get_head_iov(struct virt_queue *vq, struct iovec iov[], u16 *out, u16 *in, u16 head, struct kvm *kvm);
 u16 virt_queue__get_inout_iov(struct kvm *kvm, struct virt_queue *queue,
 			      struct iovec in_iov[], struct iovec out_iov[],
 			      u16 *in, u16 *out);

virtio/blk.c

@@ -30,7 +30,6 @@
 #define NUM_VIRT_QUEUES			1
 
 struct blk_dev_req {
-	struct list_head		list;
 	struct virt_queue		*vq;
 	struct blk_dev			*bdev;
 	struct iovec			iov[VIRTIO_BLK_QUEUE_SIZE];
@@ -57,27 +56,6 @@ struct blk_dev {
 static LIST_HEAD(bdevs);
 static int compat_id;
 
-static struct blk_dev_req *virtio_blk_req_pop(struct blk_dev *bdev)
-{
-	struct blk_dev_req *req = NULL;
-
-	mutex_lock(&bdev->req_mutex);
-	if (!list_empty(&bdev->req_list)) {
-		req = list_first_entry(&bdev->req_list, struct blk_dev_req, list);
-		list_del_init(&req->list);
-	}
-	mutex_unlock(&bdev->req_mutex);
-
-	return req;
-}
-
-static void virtio_blk_req_push(struct blk_dev *bdev, struct blk_dev_req *req)
-{
-	mutex_lock(&bdev->req_mutex);
-	list_add(&req->list, &bdev->req_list);
-	mutex_unlock(&bdev->req_mutex);
-}
-
 void virtio_blk_complete(void *param, long len)
 {
 	struct blk_dev_req *req = param;
@@ -95,8 +73,6 @@ void virtio_blk_complete(void *param, long len)
 
 	if (virtio_queue__should_signal(&bdev->vqs[queueid]))
 		bdev->vtrans.trans_ops->signal_vq(req->kvm, &bdev->vtrans, queueid);
-
-	virtio_blk_req_push(req->bdev, req);
 }
 
 static void virtio_blk_do_io_request(struct kvm *kvm, struct blk_dev_req *req)
@@ -141,15 +117,14 @@ static void virtio_blk_do_io_request(struct kvm *kvm, struct blk_dev_req *req)
 
 static void virtio_blk_do_io(struct kvm *kvm, struct virt_queue *vq, struct blk_dev *bdev)
 {
-	while (virt_queue__available(vq)) {
-		struct blk_dev_req *req = virtio_blk_req_pop(bdev);
+	struct blk_dev_req *req;
+	u16 head;
 
-		*req		= (struct blk_dev_req) {
-			.vq	= vq,
-			.bdev	= bdev,
-			.kvm	= kvm,
-		};
-		req->head = virt_queue__get_iov(vq, req->iov, &req->out, &req->in, kvm);
+	while (virt_queue__available(vq)) {
+		head		= virt_queue__pop(vq);
+		req		= &bdev->reqs[head];
+		req->head	= virt_queue__get_head_iov(vq, req->iov, &req->out, &req->in, head, kvm);
+		req->vq		= vq;
 
 		virtio_blk_do_io_request(kvm, req);
 	}
@@ -235,7 +210,7 @@ static struct virtio_ops blk_dev_virtio_ops = (struct virtio_ops) {
 void virtio_blk__init(struct kvm *kvm, struct disk_image *disk)
 {
 	struct blk_dev *bdev;
-	size_t i;
+	unsigned int i;
 
 	if (!disk)
 		return;
@@ -261,9 +236,10 @@ void virtio_blk__init(struct kvm *kvm, struct disk_image *disk)
 
 	list_add_tail(&bdev->list, &bdevs);
 
-	INIT_LIST_HEAD(&bdev->req_list);
-	for (i = 0; i < ARRAY_SIZE(bdev->reqs); i++)
-		list_add(&bdev->reqs[i].list, &bdev->req_list);
+	for (i = 0; i < ARRAY_SIZE(bdev->reqs); i++) {
+		bdev->reqs[i].bdev	= bdev;
+		bdev->reqs[i].kvm	= kvm;
+	}
 
 	disk_image__set_callback(bdev->disk, virtio_blk_complete);
 

virtio/core.c

@@ -33,18 +33,18 @@ struct vring_used_elem *virt_queue__set_used_elem(struct virt_queue *queue, u32
 	return used_elem;
 }
 
-u16 virt_queue__get_iov(struct virt_queue *queue, struct iovec iov[], u16 *out, u16 *in, struct kvm *kvm)
+u16 virt_queue__get_head_iov(struct virt_queue *vq, struct iovec iov[], u16 *out, u16 *in, u16 head, struct kvm *kvm)
 {
 	struct vring_desc *desc;
-	u16 head, idx;
+	u16 idx;
 
-	idx = head = virt_queue__pop(queue);
+	idx = head;
 	*out = *in = 0;
 
 	do {
-		desc				= virt_queue__get_desc(queue, idx);
-		iov[*out + *in].iov_base	= guest_flat_to_host(kvm, desc->addr);
-		iov[*out + *in].iov_len		= desc->len;
+		desc			 = virt_queue__get_desc(vq, idx);
+		iov[*out + *in].iov_base = guest_flat_to_host(kvm, desc->addr);
+		iov[*out + *in].iov_len	 = desc->len;
 		if (desc->flags & VRING_DESC_F_WRITE)
 			(*in)++;
 		else
@@ -58,13 +58,22 @@ u16 virt_queue__get_iov(struct virt_queue *queue, struct iovec iov[], u16 *out,
 	return head;
 }
 
+u16 virt_queue__get_iov(struct virt_queue *vq, struct iovec iov[], u16 *out, u16 *in, struct kvm *kvm)
+{
+	u16 head;
+
+	head = virt_queue__pop(vq);
+
+	return virt_queue__get_head_iov(vq, iov, out, in, head, kvm);
+}
+
 /* in and out are relative to guest */
 u16 virt_queue__get_inout_iov(struct kvm *kvm, struct virt_queue *queue,
 			      struct iovec in_iov[], struct iovec out_iov[],
 			      u16 *in, u16 *out)
 {
-	u16 head, idx;
 	struct vring_desc *desc;
+	u16 head, idx;
 
 	idx = head = virt_queue__pop(queue);
 	*out = *in = 0;
@@ -86,6 +95,7 @@ u16 virt_queue__get_inout_iov(struct kvm *kvm, struct virt_queue *queue,
 		else
 			break;
 	} while (1);
+
 	return head;
 }