Re: [PATCH V3 13/19] virtio_ring: introduce virtqueue ops

From: Michael S. Tsirkin
Date: Tue Jul 01 2025 - 02:29:14 EST

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On Mon, Jun 16, 2025 at 04:25:11PM +0800, Jason Wang wrote:
> This patch introduces virtqueue ops which is a set of the callbacks
> that will be called for different queue layout or features. This would
> help to avoid branches for split/packed and will ease the future
> implementation like in order.
>
> Note that in order to eliminate the indirect calls this patch uses
> global array of const ops to allow compiler to avoid indirect
> branches.
>
> Tested with CONFIG_MITIGATION_RETPOLINE, no performance differences
> were noticed.
>
> Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> drivers/virtio/virtio_ring.c | 172 ++++++++++++++++++++++++++---------
> 1 file changed, 129 insertions(+), 43 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index b14bbb4d6713..af32d1a1a1db 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -67,6 +67,12 @@
> #define LAST_ADD_TIME_INVALID(vq)
> #endif
>
> +enum vq_layout {
> + SPLIT = 0,
> + PACKED,
> + VQ_TYPE_MAX,
> +};
> +
> struct vring_desc_state_split {
> void *data; /* Data for callback. */
>
> @@ -159,12 +165,28 @@ struct vring_virtqueue_packed {
> size_t event_size_in_bytes;
> };
>
> +struct vring_virtqueue;
> +
> +struct virtqueue_ops {
> + int (*add)(struct vring_virtqueue *_vq, struct scatterlist *sgs[],
> + unsigned int total_sg, unsigned int out_sgs,
> + unsigned int in_sgs, void *data,
> + void *ctx, bool premapped, gfp_t gfp);
> + void *(*get)(struct vring_virtqueue *vq, unsigned int *len, void **ctx);
> + bool (*kick_prepare)(struct vring_virtqueue *vq);
> + void (*disable_cb)(struct vring_virtqueue *vq);
> + bool (*enable_cb_delayed)(struct vring_virtqueue *vq);
> + unsigned int (*enable_cb_prepare)(struct vring_virtqueue *vq);
> + bool (*poll)(const struct vring_virtqueue *vq, u16 last_used_idx);
> + void *(*detach_unused_buf)(struct vring_virtqueue *vq);
> + bool (*more_used)(const struct vring_virtqueue *vq);
> + int (*resize)(struct vring_virtqueue *vq, u32 num);
> + void (*reset)(struct vring_virtqueue *vq);
> +};
> +
> struct vring_virtqueue {
> struct virtqueue vq;
>
> - /* Is this a packed ring? */
> - bool packed_ring;
> -
> /* Is DMA API used? */
> bool use_dma_api;
>
> @@ -180,6 +202,8 @@ struct vring_virtqueue {
> /* Host publishes avail event idx */
> bool event;
>
> + enum vq_layout layout;
> +
> /* Head of free buffer list. */
> unsigned int free_head;
> /* Number we've added since last sync. */
> @@ -232,6 +256,12 @@ static void vring_free(struct virtqueue *_vq);
>
> #define to_vvq(_vq) container_of_const(_vq, struct vring_virtqueue, vq)
>
> +

Why two empty lines?

> +static inline bool virtqueue_is_packed(const struct vring_virtqueue *vq)
> +{
> + return vq->layout == PACKED;
> +}
> +

So why still have this?
Let's replace all uses with a per layout
callback. None of them seem to be on the data path.
Well, with one exception - there is
> @@ -2921,7 +3006,7 @@ u32 vring_notification_data(struct virtqueue *_vq)
> struct vring_virtqueue *vq = to_vvq(_vq);
> u16 next;
>
> - if (vq->packed_ring)
> + if (virtqueue_is_packed(vq))
> next = (vq->packed.next_avail_idx &
> ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR))) |
> vq->packed.avail_wrap_counter <<

I think it's the only one? Are we trying to save a function call there?
I am fine to keep it as in this patch, or if we do change it,
maybe with a separate patch, so if there is a regression we can bisect
more easily.

For example, for the chunk below, we could have:
.init_last_used

having said that, this patchset is already big, so let us do it with a separate patch -
but if so, the way to split would be in patch 1 to just leave vq->packed_ring
in place, gradually replace call sites, and in patch N drop
vq->packed_ring.

> static bool virtqueue_use_indirect(const struct vring_virtqueue *vq,
> unsigned int total_sg)
> {
> @@ -422,7 +452,7 @@ static void virtqueue_init(struct vring_virtqueue *vq, u32 num)
> {
> vq->vq.num_free = num;
>
> - if (vq->packed_ring)
> + if (virtqueue_is_packed(vq))
> vq->last_used_idx = 0 | (1 << VRING_PACKED_EVENT_F_WRAP_CTR);
> else
> vq->last_used_idx = 0;
> @@ -1116,6 +1146,8 @@ static int vring_alloc_queue_split(struct vring_virtqueue_split *vring_split,
> return 0;
> }
>
> +static const struct virtqueue_ops split_ops;
> +
> static struct virtqueue *__vring_new_virtqueue_split(unsigned int index,
> struct vring_virtqueue_split *vring_split,
> struct virtio_device *vdev,
> @@ -1133,7 +1165,7 @@ static struct virtqueue *__vring_new_virtqueue_split(unsigned int index,
> if (!vq)
> return NULL;
>
> - vq->packed_ring = false;
> + vq->layout = SPLIT;
> vq->vq.callback = callback;
> vq->vq.vdev = vdev;
> vq->vq.name = name;
> @@ -2076,6 +2108,8 @@ static void virtqueue_reset_packed(struct vring_virtqueue *vq)
> virtqueue_vring_init_packed(&vq->packed, !!vq->vq.callback);
> }
>
> +static const struct virtqueue_ops packed_ops;
> +
> static struct virtqueue *__vring_new_virtqueue_packed(unsigned int index,
> struct vring_virtqueue_packed *vring_packed,
> struct virtio_device *vdev,
> @@ -2106,7 +2140,7 @@ static struct virtqueue *__vring_new_virtqueue_packed(unsigned int index,
> #else
> vq->broken = false;
> #endif
> - vq->packed_ring = true;
> + vq->layout = PACKED;
> vq->dma_dev = dma_dev;
> vq->use_dma_api = vring_use_dma_api(vdev);
>
> @@ -2194,6 +2228,39 @@ static int virtqueue_resize_packed(struct vring_virtqueue *vq, u32 num)
> return -ENOMEM;
> }
>
> +static const struct virtqueue_ops split_ops = {
> + .add = virtqueue_add_split,
> + .get = virtqueue_get_buf_ctx_split,
> + .kick_prepare = virtqueue_kick_prepare_split,
> + .disable_cb = virtqueue_disable_cb_split,
> + .enable_cb_delayed = virtqueue_enable_cb_delayed_split,
> + .enable_cb_prepare = virtqueue_enable_cb_prepare_split,
> + .poll = virtqueue_poll_split,
> + .detach_unused_buf = virtqueue_detach_unused_buf_split,
> + .more_used = more_used_split,
> + .resize = virtqueue_resize_split,
> + .reset = virtqueue_reset_split,
> +};
> +
> +static const struct virtqueue_ops packed_ops = {
> + .add = virtqueue_add_packed,
> + .get = virtqueue_get_buf_ctx_packed,
> + .kick_prepare = virtqueue_kick_prepare_packed,
> + .disable_cb = virtqueue_disable_cb_packed,
> + .enable_cb_delayed = virtqueue_enable_cb_delayed_packed,
> + .enable_cb_prepare = virtqueue_enable_cb_prepare_packed,
> + .poll = virtqueue_poll_packed,
> + .detach_unused_buf = virtqueue_detach_unused_buf_packed,
> + .more_used = more_used_packed,
> + .resize = virtqueue_resize_packed,
> + .reset = virtqueue_reset_packed,
> +};
> +
> +static const struct virtqueue_ops *const all_ops[VQ_TYPE_MAX] = {
> + [SPLIT] = &split_ops,
> + [PACKED] = &packed_ops
> +};
> +
> static int virtqueue_disable_and_recycle(struct virtqueue *_vq,
> void (*recycle)(struct virtqueue *vq, void *buf))
> {
> @@ -2236,6 +2303,38 @@ static int virtqueue_enable_after_reset(struct virtqueue *_vq)
> * Generic functions and exported symbols.
> */
>
> +#define VIRTQUEUE_CALL(vq, op, ...) \
> + ({ \
> + typeof(all_ops[SPLIT]->op(vq, ##__VA_ARGS__)) ret; \

we need an empty line here, after the declaration.

> + switch (vq->layout) { \
> + case SPLIT: \
> + ret = all_ops[SPLIT]->op(vq, ##__VA_ARGS__); \
> + break; \
> + case PACKED: \
> + ret = all_ops[PACKED]->op(vq, ##__VA_ARGS__); \
> + break; \
> + default: \
> + BUG(); \
> + break; \
> + } \
> + ret; \
> +})
> +
> +#define VOID_VIRTQUEUE_CALL(vq, op, ...) \
> + ({ \
> + switch ((vq)->layout) { \
> + case SPLIT: \
> + all_ops[SPLIT]->op(vq, ##__VA_ARGS__); \
> + break; \
> + case PACKED: \
> + all_ops[PACKED]->op(vq, ##__VA_ARGS__); \
> + break; \
> + default: \
> + BUG(); \
> + break; \
> + } \
> +})
> +
> static inline int virtqueue_add(struct virtqueue *_vq,
> struct scatterlist *sgs[],
> unsigned int total_sg,
> @@ -2248,10 +2347,9 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> - return vq->packed_ring ? virtqueue_add_packed(vq, sgs, total_sg,
> - out_sgs, in_sgs, data, ctx, premapped, gfp) :
> - virtqueue_add_split(vq, sgs, total_sg,
> - out_sgs, in_sgs, data, ctx, premapped, gfp);
> + return VIRTQUEUE_CALL(vq, add, sgs, total_sg,
> + out_sgs, in_sgs, data,
> + ctx, premapped, gfp);
> }
>
> /**
> @@ -2437,8 +2535,7 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> - return vq->packed_ring ? virtqueue_kick_prepare_packed(vq) :
> - virtqueue_kick_prepare_split(vq);
> + return VIRTQUEUE_CALL(vq, kick_prepare);
> }
> EXPORT_SYMBOL_GPL(virtqueue_kick_prepare);
>
> @@ -2508,8 +2605,7 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> - return vq->packed_ring ? virtqueue_get_buf_ctx_packed(vq, len, ctx) :
> - virtqueue_get_buf_ctx_split(vq, len, ctx);
> + return VIRTQUEUE_CALL(vq, get, len, ctx);
> }
> EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
>
> @@ -2531,10 +2627,7 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> - if (vq->packed_ring)
> - virtqueue_disable_cb_packed(vq);
> - else
> - virtqueue_disable_cb_split(vq);
> + VOID_VIRTQUEUE_CALL(vq, disable_cb);
> }
> EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>
> @@ -2557,8 +2650,7 @@ unsigned int virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> if (vq->event_triggered)
> vq->event_triggered = false;
>
> - return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(vq) :
> - virtqueue_enable_cb_prepare_split(vq);
> + return VIRTQUEUE_CALL(vq, enable_cb_prepare);
> }
> EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>
> @@ -2579,8 +2671,8 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned int last_used_idx)
> return false;
>
> virtio_mb(vq->weak_barriers);
> - return vq->packed_ring ? virtqueue_poll_packed(vq, last_used_idx) :
> - virtqueue_poll_split(vq, last_used_idx);
> +
> + return VIRTQUEUE_CALL(vq, poll, last_used_idx);
> }
> EXPORT_SYMBOL_GPL(virtqueue_poll);
>
> @@ -2623,8 +2715,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> if (vq->event_triggered)
> data_race(vq->event_triggered = false);
>
> - return vq->packed_ring ? virtqueue_enable_cb_delayed_packed(vq) :
> - virtqueue_enable_cb_delayed_split(vq);
> + return VIRTQUEUE_CALL(vq, enable_cb_delayed);
> }
> EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
>
> @@ -2640,14 +2731,13 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> - return vq->packed_ring ? virtqueue_detach_unused_buf_packed(vq) :
> - virtqueue_detach_unused_buf_split(vq);
> + return VIRTQUEUE_CALL(vq, detach_unused_buf);
> }
> EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>
> static inline bool more_used(const struct vring_virtqueue *vq)
> {
> - return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
> + return VIRTQUEUE_CALL(vq, more_used);
> }
>
> /**
> @@ -2776,7 +2866,8 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
> if (!num)
> return -EINVAL;
>
> - if ((vq->packed_ring ? vq->packed.vring.num : vq->split.vring.num) == num)
> + if ((virtqueue_is_packed(vq) ? vq->packed.vring.num :
> + vq->split.vring.num) == num)
> return 0;
>
> err = virtqueue_disable_and_recycle(_vq, recycle);
> @@ -2785,10 +2876,7 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
> if (recycle_done)
> recycle_done(_vq);
>
> - if (vq->packed_ring)
> - err = virtqueue_resize_packed(vq, num);
> - else
> - err = virtqueue_resize_split(vq, num);
> + err = VIRTQUEUE_CALL(vq, resize, num);
>
> return virtqueue_enable_after_reset(_vq);
> }
> @@ -2822,10 +2910,7 @@ int virtqueue_reset(struct virtqueue *_vq,
> if (recycle_done)
> recycle_done(_vq);
>
> - if (vq->packed_ring)
> - virtqueue_reset_packed(vq);
> - else
> - virtqueue_reset_split(vq);
> + VOID_VIRTQUEUE_CALL(vq, reset);
>
> return virtqueue_enable_after_reset(_vq);
> }
> @@ -2867,7 +2952,7 @@ static void vring_free(struct virtqueue *_vq)
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> if (vq->we_own_ring) {
> - if (vq->packed_ring) {
> + if (virtqueue_is_packed(vq)) {
> vring_free_queue(vq->vq.vdev,
> vq->packed.ring_size_in_bytes,
> vq->packed.vring.desc,
> @@ -2896,7 +2981,7 @@ static void vring_free(struct virtqueue *_vq)
> vring_dma_dev(vq));
> }
> }
> - if (!vq->packed_ring) {
> + if (!virtqueue_is_packed(vq)) {
> kfree(vq->split.desc_state);
> kfree(vq->split.desc_extra);
> }
> @@ -2921,7 +3006,7 @@ u32 vring_notification_data(struct virtqueue *_vq)
> struct vring_virtqueue *vq = to_vvq(_vq);
> u16 next;
>
> - if (vq->packed_ring)
> + if (virtqueue_is_packed(vq))
> next = (vq->packed.next_avail_idx &
> ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR))) |
> vq->packed.avail_wrap_counter <<
> @@ -2974,7 +3059,8 @@ unsigned int virtqueue_get_vring_size(const struct virtqueue *_vq)
>
> const struct vring_virtqueue *vq = to_vvq(_vq);
>
> - return vq->packed_ring ? vq->packed.vring.num : vq->split.vring.num;
> + return virtqueue_is_packed(vq) ? vq->packed.vring.num :
> + vq->split.vring.num;
> }
> EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
>
> @@ -3057,7 +3143,7 @@ dma_addr_t virtqueue_get_desc_addr(const struct virtqueue *_vq)
>
> BUG_ON(!vq->we_own_ring);
>
> - if (vq->packed_ring)
> + if (virtqueue_is_packed(vq))
> return vq->packed.ring_dma_addr;
>
> return vq->split.queue_dma_addr;
> @@ -3070,7 +3156,7 @@ dma_addr_t virtqueue_get_avail_addr(const struct virtqueue *_vq)
>
> BUG_ON(!vq->we_own_ring);
>
> - if (vq->packed_ring)
> + if (virtqueue_is_packed(vq))
> return vq->packed.driver_event_dma_addr;
>
> return vq->split.queue_dma_addr +
> @@ -3084,7 +3170,7 @@ dma_addr_t virtqueue_get_used_addr(const struct virtqueue *_vq)
>
> BUG_ON(!vq->we_own_ring);
>
> - if (vq->packed_ring)
> + if (virtqueue_is_packed(vq))
> return vq->packed.device_event_dma_addr;
>
> return vq->split.queue_dma_addr +
> --
> 2.34.1

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