On Mon, Mar 24, 2025 at 3:14 PM Sahil Siddiq <icegambi...@gmail.com> wrote:
>
> Hi,
>
> I had a few queries here.
>
> On 3/24/25 7:29 PM, Sahil Siddiq 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;
>
> Since "svq->vring_packed.next_avail_idx" is part of QEMU internals and not
> stored in guest memory, no endianness conversion is required here, right?
>
Right!
> > + i = head_idx;
> > + id = svq->free_head;
> > + curr = id;
> > + *head = id;
>
> Should head be the buffer id or the idx of the descriptor ring where the
> first descriptor of a descriptor chain is inserted?
>
The buffer id of the *last* descriptor of a chain. See "2.8.6 Next
Flag: Descriptor Chaining" at [1].
> > + /* 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;
> > + } else {
> > + needs_kick = (svq->vring_packed.vring.device->flags !=
> > + cpu_to_le16(VRING_PACKED_EVENT_FLAG_DISABLE));
> > + }
> > +
> > + if (!needs_kick) {
> > + return;
> > + }
> > +
> > + event_notifier_set(&svq->hdev_kick);
> > +}
> > +
> > /**
> > * Add an element to a SVQ.
> > *
> > @@ -258,13 +356,22 @@ int vhost_svq_add(VhostShadowVirtqueue *svq, const
> > struct iovec *out_sg,
> > return -EINVAL;
> > }
> >
> > - vhost_svq_add_split(svq, out_sg, out_num, in_sg,
> > - in_num, sgs, &qemu_head);
> > + if (svq->is_packed) {
> > + vhost_svq_add_packed(svq, out_sg, out_num, in_sg,
> > + in_num, sgs, &qemu_head);
> > + } else {
> > + vhost_svq_add_split(svq, out_sg, out_num, in_sg,
> > + in_num, sgs, &qemu_head);
> > + }
> >
> > svq->num_free -= ndescs;
> > svq->desc_state[qemu_head].elem = elem;
> > svq->desc_state[qemu_head].ndescs = ndescs;
>
> *head in vhost_svq_add_packed() is stored in "qemu_head" here.
>
Sorry I don't get this, can you expand?
[1] https://docs.oasis-open.org/virtio/virtio/v1.3/virtio-v1.3.html
> > - vhost_svq_kick(svq);
> > + if (svq->is_packed) {
> > + vhost_svq_kick_packed(svq);
> > + } else {
> > + vhost_svq_kick_split(svq);
> > + }
> > return 0;
> > }
> >
>
> Thanks,
> Sahil
>
