Re: [PATCH] virtiofs: remove max_pages_limit in indirect descriptor mode

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sat, Oct 11, 2025 at 11:30:18AM +0800, Wei Gong wrote:
> From: Wei Gong <gongwei09@baidu.com>
> 
> Currently, indirect descriptor mode unnecessarily restricts the maximum
> IO size based on virtqueue vringsize. However, the indirect descriptor
> mechanism inherently supports larger IO operations by chaining descriptors.
> 
> This patch removes the artificial constraint, allowing indirect descriptor
> mode to utilize its full potential without being limited by vringsize.
> The maximum supported descriptors per IO is now determined by the indirect
> descriptor capability rather than the virtqueue size.
> 
> Signed-off-by: Wei Gong <gongwei09@baidu.com>
> ---
>  fs/fuse/virtio_fs.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 76c8fd0bfc75..c0d5db7d7504 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -12,6 +12,7 @@
>  #include <linux/memremap.h>
>  #include <linux/module.h>
>  #include <linux/virtio.h>
> +#include <linux/virtio_ring.h>
>  #include <linux/virtio_fs.h>
>  #include <linux/delay.h>
>  #include <linux/fs_context.h>
> @@ -1701,9 +1702,11 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>  	fc->sync_fs = true;
>  	fc->use_pages_for_kvec_io = true;
>  
> -	/* Tell FUSE to split requests that exceed the virtqueue's size */
> -	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
> -				    virtqueue_size - FUSE_HEADER_OVERHEAD);
> +	if (!virtio_has_feature(fs->vqs[VQ_REQUEST].vq->vdev, VIRTIO_RING_F_INDIRECT_DESC)) {
> +		/* Tell FUSE to split requests that exceed the virtqueue's size */
> +		fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
> +						virtqueue_size - FUSE_HEADER_OVERHEAD);
> +	}

The VIRTIO 1.3 specification defines the maximum descriptor chain length
as follows
(https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-9200019):

  The number of descriptors in the table is defined by the queue size for this virtqueue: this is the maximum possible descriptor chain length.

The driver requirements for indirect descriptors say
(https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-9200019):

  A driver MUST NOT create a descriptor chain longer than allowed by the device.

My interpretation is that this patch violates the specification because
it allows descriptor chains that exceed the maximum possible descriptor
chain length.

Device implementations are not required to enforce this limit, so you
may not see issues when testing. Nevertheless, this patch has the
potential to break other device implementations though that work fine
today, so it doesn't seem safe to merge this patch in its current form.

I have CCed Michael Tsirkin in case he has thoughts. It would be nice to
boost performance by allowing longer I/O requests, but the driver must
comply with the VIRTIO specification.

Thanks,
Stefan

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