[Virtio-fs] [PATCH v2 1/3] virtio: turn VIRTQUEUE_MAX_SIZE into a variable
Stefan Hajnoczi
stefanha at redhat.com
Tue Oct 5 12:45:56 UTC 2021
On Mon, Oct 04, 2021 at 09:38:04PM +0200, Christian Schoenebeck wrote:
> Refactor VIRTQUEUE_MAX_SIZE to effectively become a runtime
> variable per virtio user.
virtio user == virtio device model?
>
> Reasons:
>
> (1) VIRTQUEUE_MAX_SIZE should reflect the absolute theoretical
> maximum queue size possible. Which is actually the maximum
> queue size allowed by the virtio protocol. The appropriate
> value for VIRTQUEUE_MAX_SIZE would therefore be 32768:
>
> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-240006
>
> Apparently VIRTQUEUE_MAX_SIZE was instead defined with a
> more or less arbitrary value of 1024 in the past, which
> limits the maximum transfer size with virtio to 4M
> (more precise: 1024 * PAGE_SIZE, with the latter typically
> being 4k).
Being equal to IOV_MAX is a likely reason. Buffers with more iovecs than
that cannot be passed to host system calls (sendmsg(2), pwritev(2),
etc).
> (2) Additionally the current value of 1024 poses a hidden limit,
> invisible to guest, which causes a system hang with the
> following QEMU error if guest tries to exceed it:
>
> virtio: too many write descriptors in indirect table
I don't understand this point. 2.6.5 The Virtqueue Descriptor Table says:
The number of descriptors in the table is defined by the queue size for this virtqueue: this is the maximum possible descriptor chain length.
and 2.6.5.3.1 Driver Requirements: Indirect Descriptors says:
A driver MUST NOT create a descriptor chain longer than the Queue Size of the device.
Do you mean a broken/malicious guest driver that is violating the spec?
That's not a hidden limit, it's defined by the spec.
> (3) Unfortunately not all virtio users in QEMU would currently
> work correctly with the new value of 32768.
>
> So let's turn this hard coded global value into a runtime
> variable as a first step in this commit, configurable for each
> virtio user by passing a corresponding value with virtio_init()
> call.
virtio_add_queue() already has an int queue_size argument, why isn't
that enough to deal with the maximum queue size? There's probably a good
reason for it, but please include it in the commit description.
>
> Signed-off-by: Christian Schoenebeck <qemu_oss at crudebyte.com>
> ---
> hw/9pfs/virtio-9p-device.c | 3 ++-
> hw/block/vhost-user-blk.c | 2 +-
> hw/block/virtio-blk.c | 3 ++-
> hw/char/virtio-serial-bus.c | 2 +-
> hw/display/virtio-gpu-base.c | 2 +-
> hw/input/virtio-input.c | 2 +-
> hw/net/virtio-net.c | 15 ++++++++-------
> hw/scsi/virtio-scsi.c | 2 +-
> hw/virtio/vhost-user-fs.c | 2 +-
> hw/virtio/vhost-user-i2c.c | 3 ++-
> hw/virtio/vhost-vsock-common.c | 2 +-
> hw/virtio/virtio-balloon.c | 4 ++--
> hw/virtio/virtio-crypto.c | 3 ++-
> hw/virtio/virtio-iommu.c | 2 +-
> hw/virtio/virtio-mem.c | 2 +-
> hw/virtio/virtio-pmem.c | 2 +-
> hw/virtio/virtio-rng.c | 2 +-
> hw/virtio/virtio.c | 35 +++++++++++++++++++++++-----------
> include/hw/virtio/virtio.h | 5 ++++-
> 19 files changed, 57 insertions(+), 36 deletions(-)
>
> diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> index 54ee93b71f..cd5d95dd51 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -216,7 +216,8 @@ static void virtio_9p_device_realize(DeviceState *dev, Error **errp)
> }
>
> v->config_size = sizeof(struct virtio_9p_config) + strlen(s->fsconf.tag);
> - virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, v->config_size);
> + virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, v->config_size,
> + VIRTQUEUE_MAX_SIZE);
> v->vq = virtio_add_queue(vdev, MAX_REQ, handle_9p_output);
> }
>
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index ba13cb87e5..336f56705c 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -491,7 +491,7 @@ static void vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
> }
>
> virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK,
> - sizeof(struct virtio_blk_config));
> + sizeof(struct virtio_blk_config), VIRTQUEUE_MAX_SIZE);
>
> s->virtqs = g_new(VirtQueue *, s->num_queues);
> for (i = 0; i < s->num_queues; i++) {
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index f139cd7cc9..9c0f46815c 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -1213,7 +1213,8 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
>
> virtio_blk_set_config_size(s, s->host_features);
>
> - virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, s->config_size);
> + virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, s->config_size,
> + VIRTQUEUE_MAX_SIZE);
>
> s->blk = conf->conf.blk;
> s->rq = NULL;
> diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
> index f01ec2137c..9ad9111115 100644
> --- a/hw/char/virtio-serial-bus.c
> +++ b/hw/char/virtio-serial-bus.c
> @@ -1045,7 +1045,7 @@ static void virtio_serial_device_realize(DeviceState *dev, Error **errp)
> config_size = offsetof(struct virtio_console_config, emerg_wr);
> }
> virtio_init(vdev, "virtio-serial", VIRTIO_ID_CONSOLE,
> - config_size);
> + config_size, VIRTQUEUE_MAX_SIZE);
>
> /* Spawn a new virtio-serial bus on which the ports will ride as devices */
> qbus_init(&vser->bus, sizeof(vser->bus), TYPE_VIRTIO_SERIAL_BUS,
> diff --git a/hw/display/virtio-gpu-base.c b/hw/display/virtio-gpu-base.c
> index c8da4806e0..20b06a7adf 100644
> --- a/hw/display/virtio-gpu-base.c
> +++ b/hw/display/virtio-gpu-base.c
> @@ -171,7 +171,7 @@ virtio_gpu_base_device_realize(DeviceState *qdev,
>
> g->virtio_config.num_scanouts = cpu_to_le32(g->conf.max_outputs);
> virtio_init(VIRTIO_DEVICE(g), "virtio-gpu", VIRTIO_ID_GPU,
> - sizeof(struct virtio_gpu_config));
> + sizeof(struct virtio_gpu_config), VIRTQUEUE_MAX_SIZE);
>
> if (virtio_gpu_virgl_enabled(g->conf)) {
> /* use larger control queue in 3d mode */
> diff --git a/hw/input/virtio-input.c b/hw/input/virtio-input.c
> index 54bcb46c74..345eb2cce7 100644
> --- a/hw/input/virtio-input.c
> +++ b/hw/input/virtio-input.c
> @@ -258,7 +258,7 @@ static void virtio_input_device_realize(DeviceState *dev, Error **errp)
> assert(vinput->cfg_size <= sizeof(virtio_input_config));
>
> virtio_init(vdev, "virtio-input", VIRTIO_ID_INPUT,
> - vinput->cfg_size);
> + vinput->cfg_size, VIRTQUEUE_MAX_SIZE);
> vinput->evt = virtio_add_queue(vdev, 64, virtio_input_handle_evt);
> vinput->sts = virtio_add_queue(vdev, 64, virtio_input_handle_sts);
> }
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index f205331dcf..f74b5f6268 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1746,9 +1746,9 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
> VirtIONet *n = qemu_get_nic_opaque(nc);
> VirtIONetQueue *q = virtio_net_get_subqueue(nc);
> VirtIODevice *vdev = VIRTIO_DEVICE(n);
> - VirtQueueElement *elems[VIRTQUEUE_MAX_SIZE];
> - size_t lens[VIRTQUEUE_MAX_SIZE];
> - struct iovec mhdr_sg[VIRTQUEUE_MAX_SIZE];
> + VirtQueueElement *elems[vdev->queue_max_size];
> + size_t lens[vdev->queue_max_size];
> + struct iovec mhdr_sg[vdev->queue_max_size];
Can you make this value per-vq instead of per-vdev since virtqueues can
have different queue sizes?
The same applies to the rest of this patch. Anything using
vdev->queue_max_size should probably use vq->vring.num instead.
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