Re: [PATCH 2/2] vdpa: send CVQ state load commands in parallel

[Eugenio Perez Martin]

On Wed, Apr 19, 2023 at 1:50 PM Hawkins Jiawei <yin31149@gmail.com> wrote:
>
> This patch introduces the vhost_vdpa_net_cvq_add() and
> refactors the vhost_vdpa_net_load*(), so that QEMU can
> send CVQ state load commands in parallel.
>
> To be more specific, this patch introduces vhost_vdpa_net_cvq_add()
> to add SVQ control commands to SVQ and kick the device,
> but does not poll the device used buffers. QEMU will not
> poll and check the device used buffers in vhost_vdpa_net_load()
> until all CVQ state load commands have been sent to the device.
>
> What's more, in order to avoid buffer overwriting caused by
> using `svq->cvq_cmd_out_buffer` and `svq->status` as the
> buffer for all CVQ state load commands when sending
> CVQ state load commands in parallel, this patch introduces
> `out_cursor` and `in_cursor` in vhost_vdpa_net_load(),
> pointing to the available buffer for in descriptor and
> out descriptor, so that different CVQ state load commands can
> use their unique buffer.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1578
> Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
> ---
>  net/vhost-vdpa.c | 137 +++++++++++++++++++++++++++++++++++------------
>  1 file changed, 102 insertions(+), 35 deletions(-)
>
> diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
> index 10804c7200..d1f7113992 100644
> --- a/net/vhost-vdpa.c
> +++ b/net/vhost-vdpa.c
> @@ -590,6 +590,44 @@ static void vhost_vdpa_net_cvq_stop(NetClientState *nc)
>      vhost_vdpa_net_client_stop(nc);
>  }
>
> +/**
> + * vhost_vdpa_net_cvq_add() adds SVQ control commands to SVQ,
> + * kicks the device but does not poll the device used buffers.
> + *
> + * Return the length of the device used buffers.
> + */
> +static ssize_t vhost_vdpa_net_cvq_add(VhostVDPAState *s,
> +                                void **out_cursor, size_t out_len,
> +                                void **in_cursor, size_t in_len)
> +{
> +    /* Buffers for the device */
> +    const struct iovec out = {
> +        .iov_base = *out_cursor,
> +        .iov_len = out_len,
> +    };
> +    const struct iovec in = {
> +        .iov_base = *in_cursor,
> +        .iov_len = in_len,
> +    };
> +    VhostShadowVirtqueue *svq = g_ptr_array_index(s->vhost_vdpa.shadow_vqs, 
> 0);
> +    int r;
> +
> +    r = vhost_svq_add(svq, &out, 1, &in, 1, NULL);
> +    if (unlikely(r != 0)) {
> +        if (unlikely(r == -ENOSPC)) {
> +            qemu_log_mask(LOG_GUEST_ERROR, "%s: No space on device queue\n",
> +                          __func__);
> +        }
> +        return r;
> +    }
> +
> +    /* Update the cursor */
> +    *out_cursor += out_len;
> +    *in_cursor += in_len;
> +
> +    return in_len;
> +}
> +
>  /**
>   * vhost_vdpa_net_cvq_add_and_wait() adds SVQ control commands to SVQ,
>   * kicks the device and polls the device used buffers.
> @@ -628,69 +666,64 @@ static ssize_t 
> vhost_vdpa_net_cvq_add_and_wait(VhostVDPAState *s,
>      return vhost_svq_poll(svq);
>  }
>
> -static ssize_t vhost_vdpa_net_load_cmd(VhostVDPAState *s, uint8_t class,
> -                                       uint8_t cmd, const void *data,
> -                                       size_t data_size)
> +static ssize_t vhost_vdpa_net_load_cmd(VhostVDPAState *s,
> +                                void **out_cursor, uint8_t class, uint8_t 
> cmd,
> +                                const void *data, size_t data_size,
> +                                void **in_cursor)
>  {
>      const struct virtio_net_ctrl_hdr ctrl = {
>          .class = class,
>          .cmd = cmd,
>      };
>
> -    assert(data_size < vhost_vdpa_net_cvq_cmd_page_len() - sizeof(ctrl));
> +    assert(sizeof(ctrl) < vhost_vdpa_net_cvq_cmd_page_len() -
> +                          (*out_cursor - s->cvq_cmd_out_buffer));
> +    assert(data_size < vhost_vdpa_net_cvq_cmd_page_len() - sizeof(ctrl) -
> +                       (*out_cursor - s->cvq_cmd_out_buffer));
>
> -    memcpy(s->cvq_cmd_out_buffer, &ctrl, sizeof(ctrl));
> -    memcpy(s->cvq_cmd_out_buffer + sizeof(ctrl), data, data_size);
> +    memcpy(*out_cursor, &ctrl, sizeof(ctrl));
> +    memcpy(*out_cursor + sizeof(ctrl), data, data_size);
>
> -    return vhost_vdpa_net_cvq_add_and_wait(s, sizeof(ctrl) + data_size,
> -                                  sizeof(virtio_net_ctrl_ack));
> +    return vhost_vdpa_net_cvq_add(s, out_cursor, sizeof(ctrl) + data_size,
> +                                  in_cursor, sizeof(virtio_net_ctrl_ack));
>  }
>
> -static int vhost_vdpa_net_load_mac(VhostVDPAState *s, const VirtIONet *n)
> +static ssize_t vhost_vdpa_net_load_mac(VhostVDPAState *s, const VirtIONet *n,
> +                                   void **out_cursor, void **in_cursor)
>  {
>      uint64_t features = n->parent_obj.guest_features;
>      if (features & BIT_ULL(VIRTIO_NET_F_CTRL_MAC_ADDR)) {
> -        ssize_t dev_written = vhost_vdpa_net_load_cmd(s, VIRTIO_NET_CTRL_MAC,
> -                                                  
> VIRTIO_NET_CTRL_MAC_ADDR_SET,
> -                                                  n->mac, sizeof(n->mac));
> -        if (unlikely(dev_written < 0)) {
> -            return dev_written;
> -        }
> -
> -        return *s->status != VIRTIO_NET_OK;
> +        return vhost_vdpa_net_load_cmd(s, out_cursor, VIRTIO_NET_CTRL_MAC,
> +                                       VIRTIO_NET_CTRL_MAC_ADDR_SET,
> +                                       n->mac, sizeof(n->mac), in_cursor);
>      }
>
>      return 0;
>  }
>
> -static int vhost_vdpa_net_load_mq(VhostVDPAState *s,
> -                                  const VirtIONet *n)
> +static ssize_t vhost_vdpa_net_load_mq(VhostVDPAState *s, const VirtIONet *n,
> +                                  void **out_cursor, void **in_cursor)
>  {
>      struct virtio_net_ctrl_mq mq;
>      uint64_t features = n->parent_obj.guest_features;
> -    ssize_t dev_written;
>
>      if (!(features & BIT_ULL(VIRTIO_NET_F_MQ))) {
>          return 0;
>      }
>
> -    mq.virtqueue_pairs = cpu_to_le16(n->curr_queue_pairs);

So where is mq filled now?

> -    dev_written = vhost_vdpa_net_load_cmd(s, VIRTIO_NET_CTRL_MQ,
> -                                          VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET, 
> &mq,
> -                                          sizeof(mq));
> -    if (unlikely(dev_written < 0)) {
> -        return dev_written;
> -    }
> -
> -    return *s->status != VIRTIO_NET_OK;
> +    return vhost_vdpa_net_load_cmd(s, out_cursor, VIRTIO_NET_CTRL_MQ,
> +                                   VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET,
> +                                   &mq, sizeof(mq), in_cursor);
>  }
>
>  static int vhost_vdpa_net_load(NetClientState *nc)
>  {
>      VhostVDPAState *s = DO_UPCAST(VhostVDPAState, nc, nc);
> +    VhostShadowVirtqueue *svq;
> +    void *out_cursor, *in_cursor;
>      struct vhost_vdpa *v = &s->vhost_vdpa;
>      const VirtIONet *n;
> -    int r;
> +    ssize_t need_poll_len = 0, r, dev_written;
>
>      assert(nc->info->type == NET_CLIENT_DRIVER_VHOST_VDPA);
>
> @@ -699,16 +732,50 @@ static int vhost_vdpa_net_load(NetClientState *nc)
>      }
>
>      n = VIRTIO_NET(v->dev->vdev);
> -    r = vhost_vdpa_net_load_mac(s, n);
> +

Extra newline.

> +    need_poll_len = 0;

Maybe we can call it "cmds_in_flight" or something similar? It is not a length.

> +    out_cursor = s->cvq_cmd_out_buffer;
> +    in_cursor = s->status;
> +
> +    r = vhost_vdpa_net_load_mac(s, n, &out_cursor, &in_cursor);
>      if (unlikely(r < 0)) {
> -        return r;
> +        goto load_err;
> +    }
> +    need_poll_len += r;
> +
> +    r = vhost_vdpa_net_load_mq(s, n, &out_cursor, &in_cursor);
> +    if (unlikely(r < 0)) {
> +        goto load_err;
> +    }
> +    need_poll_len += r;
> +
> +load_err:
> +    /* Poll for all SVQ control commands used buffer from device */
> +    svq = g_ptr_array_index(s->vhost_vdpa.shadow_vqs, 0);
> +    while (need_poll_len > 0) {
> +        /*
> +         * We can poll here since we've had BQL from the time we sent the
> +         * descriptor. Also, we need to take the answer before SVQ pulls
> +         * by itself, when BQL is released
> +         */
> +        dev_written = vhost_svq_poll(svq);

If vhost_svq_poll returns 0 we must return an error from this code.
Otherwise this will be an infinite loop.

> +        need_poll_len -= dev_written;

On the other hand, vhost_svq_poll returns 1 because that is the length
written in the "in" buffer, but it is not obvious here. It is more
clear if we just do need_poll_len-- here.

>      }
> -    r = vhost_vdpa_net_load_mq(s, n);
> -    if (unlikely(r)) {
> +
> +    /* If code comes from `load_err` label, then we should return directly */
> +    if (unlikely(r < 0)) {

If this function returns a failure we can avoid the sending of the
queued commands, as the caller must cancel the start of the device
anyway. We can return directly from the failure in
vhost_vdpa_net_load_* and avoid the label.

>          return r;
>      }
>
> -    return 0;
> +    /* Check the used buffer from device */
> +    for (virtio_net_ctrl_ack * status = s->status; (void *)status < 
> in_cursor;

in_cursor can be a virtio_net_ctrl_ack so we don't need the casting here.

> +         ++status) {
> +        if (*status != VIRTIO_NET_OK) {
> +            ++r;
> +        }
> +    }
> +
> +    return r;

Although the caller is fine with >=0, I think we should keep the 0 ==
success. The number of commands delivered does not make a lot of sense
for the callers, just if the call succeeded or not.

Thanks!

>  }
>
>  static NetClientInfo net_vhost_vdpa_cvq_info = {
> --
> 2.25.1
>