On Fri, Mar 28, 2025 at 6:10 AM Sahil Siddiq <icegambi...@gmail.com> wrote:
>
> Hi,
>
> On 3/26/25 5:32 PM, Eugenio Perez Martin wrote:
> > On Mon, Mar 24, 2025 at 3:00 PM Sahil Siddiq <icegambi...@gmail.com> wrote:
> >>
> >> Implement the insertion of available buffers in the descriptor area of
> >> packed shadow virtqueues. It takes into account descriptor chains, but
> >> does not consider indirect descriptors.
> >>
> >> Enable the packed SVQ to forward the descriptors to the device.
> >>
> >> Signed-off-by: Sahil Siddiq <sahil...@proton.me>
> >> ---
> >> Changes from v4 -> v5:
> >> - This was commit #2 in v4. This has been reordered to commit #3
> >> based on review comments.
> >> - vhost-shadow-virtqueue.c:
> >> (vhost_svq_valid_features): Move addition of enums to commit #6
> >> based on review comments.
> >> (vhost_svq_add_packed): Set head_idx to buffer id instead of vring's
> >> index.
> >> (vhost_svq_kick): Split into vhost_svq_kick_split and
> >> vhost_svq_kick_packed.
> >> (vhost_svq_add): Use new vhost_svq_kick_* functions.
> >>
> >> hw/virtio/vhost-shadow-virtqueue.c | 117 +++++++++++++++++++++++++++--
> >> 1 file changed, 112 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/hw/virtio/vhost-shadow-virtqueue.c
> >> b/hw/virtio/vhost-shadow-virtqueue.c
> >> index 4f74ad402a..6e16cd4bdf 100644
> >> --- a/hw/virtio/vhost-shadow-virtqueue.c
> >> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> >> @@ -193,10 +193,83 @@ static void vhost_svq_add_split(VhostShadowVirtqueue
> >> *svq,
> >> /* Update the avail index after write the descriptor */
> >> smp_wmb();
> >> avail->idx = cpu_to_le16(svq->shadow_avail_idx);
> >> +}
> >> +
> >> +/**
> >> + * Write descriptors to SVQ packed vring
> >> + *
> >> + * @svq: The shadow virtqueue
> >> + * @out_sg: The iovec to the guest
> >> + * @out_num: Outgoing iovec length
> >> + * @in_sg: The iovec from the guest
> >> + * @in_num: Incoming iovec length
> >> + * @sgs: Cache for hwaddr
> >> + * @head: Saves current free_head
> >> + */
> >> +static void vhost_svq_add_packed(VhostShadowVirtqueue *svq,
> >> + const struct iovec *out_sg, size_t
> >> out_num,
> >> + const struct iovec *in_sg, size_t in_num,
> >> + hwaddr *sgs, unsigned *head)
> >> +{
> >> + uint16_t id, curr, i, head_flags = 0, head_idx;
> >> + size_t num = out_num + in_num;
> >> + unsigned n;
> >> +
> >> + struct vring_packed_desc *descs = svq->vring_packed.vring.desc;
> >> +
> >> + head_idx = svq->vring_packed.next_avail_idx;
> >> + i = head_idx;
> >> + id = svq->free_head;
> >> + curr = id;
> >> + *head = id;
> >> +
> >> + /* Write descriptors to SVQ packed vring */
> >> + for (n = 0; n < num; n++) {
> >> + uint16_t flags = cpu_to_le16(svq->vring_packed.avail_used_flags |
> >> + (n < out_num ? 0 :
> >> VRING_DESC_F_WRITE) |
> >> + (n + 1 == num ? 0 :
> >> VRING_DESC_F_NEXT));
> >> + if (i == head_idx) {
> >> + head_flags = flags;
> >> + } else {
> >> + descs[i].flags = flags;
> >> + }
> >> +
> >> + descs[i].addr = cpu_to_le64(sgs[n]);
> >> + descs[i].id = id;
> >> + if (n < out_num) {
> >> + descs[i].len = cpu_to_le32(out_sg[n].iov_len);
> >> + } else {
> >> + descs[i].len = cpu_to_le32(in_sg[n - out_num].iov_len);
> >> + }
> >> +
> >> + curr = cpu_to_le16(svq->desc_next[curr]);
> >> +
> >> + if (++i >= svq->vring_packed.vring.num) {
> >> + i = 0;
> >> + svq->vring_packed.avail_used_flags ^=
> >> + 1 << VRING_PACKED_DESC_F_AVAIL |
> >> + 1 << VRING_PACKED_DESC_F_USED;
> >> + }
> >> + }
> >>
> >> + if (i <= head_idx) {
> >> + svq->vring_packed.avail_wrap_counter ^= 1;
> >> + }
> >> +
> >> + svq->vring_packed.next_avail_idx = i;
> >> + svq->shadow_avail_idx = i;
> >> + svq->free_head = curr;
> >> +
> >> + /*
> >> + * A driver MUST NOT make the first descriptor in the list
> >> + * available before all subsequent descriptors comprising
> >> + * the list are made available.
> >> + */
> >> + smp_wmb();
> >> + svq->vring_packed.vring.desc[head_idx].flags = head_flags;
> >> }
> >>
> >> -static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> >> +static void vhost_svq_kick_split(VhostShadowVirtqueue *svq)
> >> {
> >> bool needs_kick;
> >>
> >> @@ -209,7 +282,8 @@ static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> >> if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >> uint16_t avail_event = le16_to_cpu(
> >> *(uint16_t *)(&svq->vring.used->ring[svq->vring.num]));
> >> - needs_kick = vring_need_event(avail_event, svq->shadow_avail_idx,
> >> svq->shadow_avail_idx - 1);
> >> + needs_kick = vring_need_event(avail_event, svq->shadow_avail_idx,
> >> + svq->shadow_avail_idx - 1);
> >> } else {
> >> needs_kick =
> >> !(svq->vring.used->flags &
> >> cpu_to_le16(VRING_USED_F_NO_NOTIFY));
> >> @@ -222,6 +296,30 @@ static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> >> event_notifier_set(&svq->hdev_kick);
> >> }
> >>
> >> +static void vhost_svq_kick_packed(VhostShadowVirtqueue *svq)
> >> +{
> >> + bool needs_kick;
> >> +
> >> + /*
> >> + * We need to expose the available array entries before checking
> >> + * notification suppressions.
> >> + */
> >> + smp_mb();
> >> +
> >> + if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >> + return;
> >
> > It's weird SVQ does not need to kick if _F_EVENT_IDX. This should have
> > code checking the device ring flags etc.
> >
>
> Right, I haven't implemented this yet. Since the current implementation is
> being tested with event_idx=off (points 3 and 4 of the roadmap [1]), I thought
> I would leave this for later.
>
> Maybe I can add a comment in the implementation explaining this.
>
Sure that's fine, and probably even better than trying to address
everything in one shot :) Can you add a TODO in each place you
identify so we're sure we don't miss any?
> Thanks,
> Sahil
>
> [1] https://wiki.qemu.org/Internships/ProjectIdeas/PackedShadowVirtqueue
>
