On Wed, Sep 24, 2025 at 01:38:03PM +0800, Jason Wang wrote: > On Mon, Sep 22, 2025 at 2:24 AM Michael S. Tsirkin <[email protected]> wrote: > > > > On Fri, Sep 19, 2025 at 03:31:54PM +0800, Jason Wang wrote: > > > This patch implements in order support for both split virtqueue and > > > packed virtqueue. Perfomance could be gained for the device where the > > > memory access could be expensive (e.g vhost-net or a real PCI device): > > > > > > Benchmark with KVM guest: > > > > > > Vhost-net on the host: (pktgen + XDP_DROP): > > > > > > in_order=off | in_order=on | +% > > > TX: 5.20Mpps | 6.20Mpps | +19% > > > RX: 3.47Mpps | 3.61Mpps | + 4% > > > > > > Vhost-user(testpmd) on the host: (pktgen/XDP_DROP): > > > > > > For split virtqueue: > > > > > > in_order=off | in_order=on | +% > > > TX: 5.60Mpps | 5.60Mpps | +0.0% > > > RX: 9.16Mpps | 9.61Mpps | +4.9% > > > > > > For packed virtqueue: > > > > > > in_order=off | in_order=on | +% > > > TX: 5.60Mpps | 5.70Mpps | +1.7% > > > RX: 10.6Mpps | 10.8Mpps | +1.8% > > > > > > Benchmark also shows no performance impact for in_order=off for queue > > > size with 256 and 1024. > > > > > > Signed-off-by: Jason Wang <[email protected]> > > > Signed-off-by: Michael S. Tsirkin <[email protected]> > > > --- > > > drivers/virtio/virtio_ring.c | 421 +++++++++++++++++++++++++++++++++-- > > > 1 file changed, 401 insertions(+), 20 deletions(-) > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > index b700aa3e56c3..c00b5e57f2fc 100644 > > > --- a/drivers/virtio/virtio_ring.c > > > +++ b/drivers/virtio/virtio_ring.c > > > @@ -70,6 +70,8 @@ > > > enum vq_layout { > > > SPLIT = 0, > > > PACKED, > > > + SPLIT_IN_ORDER, > > > + PACKED_IN_ORDER, > > > VQ_TYPE_MAX, > > > }; > > > > > > @@ -80,6 +82,7 @@ struct vring_desc_state_split { > > > * allocated together. So we won't stress more to the memory > > > allocator. > > > */ > > > struct vring_desc *indir_desc; > > > + u32 total_len; /* Buffer Length */ > > > }; > > > > > > struct vring_desc_state_packed { > > > @@ -91,6 +94,7 @@ struct vring_desc_state_packed { > > > struct vring_packed_desc *indir_desc; > > > u16 num; /* Descriptor list length. */ > > > u16 last; /* The last desc state in a list. */ > > > + u32 total_len; /* Buffer Length */ > > > }; > > > > > > struct vring_desc_extra { > > > @@ -206,6 +210,17 @@ struct vring_virtqueue { > > > > > > /* Head of free buffer list. */ > > > unsigned int free_head; > > > + > > > + /* > > > + * With IN_ORDER, devices write a single used ring entry with > > > + * the id corresponding to the head entry of the descriptor chain > > > + * describing the last buffer in the batch > > > + */ > > > + struct used_entry { > > > + u32 id; > > > + u32 len; > > > + } batch_last; > > > + > > > /* Number we've added since last sync. */ > > > unsigned int num_added; > > > > > > @@ -258,7 +273,12 @@ static void vring_free(struct virtqueue *_vq); > > > > > > static inline bool virtqueue_is_packed(const struct vring_virtqueue *vq) > > > { > > > - return vq->layout == PACKED; > > > + return vq->layout == PACKED || vq->layout == PACKED_IN_ORDER; > > > +} > > > + > > > +static inline bool virtqueue_is_in_order(const struct vring_virtqueue > > > *vq) > > > +{ > > > + return vq->layout == SPLIT_IN_ORDER || vq->layout == > > > PACKED_IN_ORDER; > > > } > > > > > > static bool virtqueue_use_indirect(const struct vring_virtqueue *vq, > > > @@ -575,6 +595,8 @@ static inline int virtqueue_add_split(struct > > > vring_virtqueue *vq, > > > struct scatterlist *sg; > > > struct vring_desc *desc; > > > unsigned int i, n, avail, descs_used, err_idx, c = 0; > > > + /* Total length for in-order */ > > > + unsigned int total_len = 0; > > > int head; > > > bool indirect; > > > > > > @@ -646,6 +668,7 @@ static inline int virtqueue_add_split(struct > > > vring_virtqueue *vq, > > > ++c == total_sg ? > > > 0 : VRING_DESC_F_NEXT, > > > premapped); > > > + total_len += len; > > > } > > > } > > > for (; n < (out_sgs + in_sgs); n++) { > > > @@ -663,6 +686,7 @@ static inline int virtqueue_add_split(struct > > > vring_virtqueue *vq, > > > i, addr, len, > > > (++c == total_sg ? 0 : VRING_DESC_F_NEXT) | > > > VRING_DESC_F_WRITE, premapped); > > > + total_len += len; > > > } > > > } > > > > > > @@ -685,7 +709,12 @@ static inline int virtqueue_add_split(struct > > > vring_virtqueue *vq, > > > vq->vq.num_free -= descs_used; > > > > > > /* Update free pointer */ > > > - if (indirect) > > > + if (virtqueue_is_in_order(vq)) { > > > + vq->free_head += descs_used; > > > + if (vq->free_head >= vq->split.vring.num) > > > + vq->free_head -= vq->split.vring.num; > > > + vq->split.desc_state[head].total_len = total_len;; > > > + } else if (indirect) > > > vq->free_head = vq->split.desc_extra[head].next; > > > else > > > vq->free_head = i; > > > @@ -858,6 +887,14 @@ static bool more_used_split(const struct > > > vring_virtqueue *vq) > > > return virtqueue_poll_split(vq, vq->last_used_idx); > > > } > > > > > > +static bool more_used_split_in_order(const struct vring_virtqueue *vq) > > > +{ > > > + if (vq->batch_last.id != vq->packed.vring.num) > > > + return true; > > > > Hmm why ->packed? > > Right, it's a bug. Let me fix that. > > > > > This is actually a problem in this approach, kinda easy to get confused > > which variant to call where. > > Probably, but we have been doing this since the introduction of packed > virtqueue. > > > > > Worth thinking how to fix this. > > > > Yes, but I think this series improves this by introducing the > virtqueue ops. Optimization could be done on top. > > For example, having separate files for packed and split with private > structure. > > Thanks
sure Besides, LLMs are getting good at catching this kind of bug. It might be enough to just add a file under Documentation/ describing the rules, at this point, plus a code comment pointing there. -- MST
