Subject: Re: [PATCH] vhost/net: Set num_buffers for virtio 1.0

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Nov 2024 03:54:26 -0500

On Sun, Sep 15, 2024 at 10:35:53AM +0900, Akihiko Odaki wrote:
> The specification says the device MUST set num_buffers to 1 if
> VIRTIO_NET_F_MRG_RXBUF has not been negotiated.
> 
> Fixes: 41e3e42108bc ("vhost/net: enable virtio 1.0")
> Signed-off-by: Akihiko Odaki <[email-masked]>

True, this is out of spec. But, qemu is also out of spec :(

Given how many years this was out there, I wonder whether
we should just fix the spec, instead of changing now.

Jason, what's your take?

> ---
>  drivers/vhost/net.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f16279351db5..d4d97fa9cc8f 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1107,6 +1107,7 @@ static void handle_rx(struct vhost_net *net)
>  	size_t vhost_hlen, sock_hlen;
>  	size_t vhost_len, sock_len;
>  	bool busyloop_intr = false;
> +	bool set_num_buffers;
>  	struct socket *sock;
>  	struct iov_iter fixup;
>  	__virtio16 num_buffers;
> @@ -1129,6 +1130,8 @@ static void handle_rx(struct vhost_net *net)
>  	vq_log = unlikely(vhost_has_feature(vq, VHOST_F_LOG_ALL)) ?
>  		vq->log : NULL;
>  	mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
> +	set_num_buffers = mergeable ||
> +			  vhost_has_feature(vq, VIRTIO_F_VERSION_1);
>  
>  	do {
>  		sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
> @@ -1205,7 +1208,7 @@ static void handle_rx(struct vhost_net *net)
>  		/* TODO: Should check and handle checksum. */
>  
>  		num_buffers = cpu_to_vhost16(vq, headcount);
> -		if (likely(mergeable) &&
> +		if (likely(set_num_buffers) &&
>  		    copy_to_iter(&num_buffers, sizeof num_buffers,
>  				 &fixup) != sizeof num_buffers) {
>  			vq_err(vq, "Failed num_buffers write");
> 
> ---
> base-commit: 46a0057a5853cbdb58211c19e89ba7777dc6fd50
> change-id: 20240908-v1-90fc83ff8b09
> 
> Best regards,
> -- 
> Akihiko Odaki <[email-masked]>