Re: virtio_net: support device stats

[Xuan Zhuo]

On Mon, 14 Oct 2024 05:47:41 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> On Mon, Oct 14, 2024 at 10:39:26AM +0100, Colin King (gmail) wrote:
> > Hi,
> >
> > Static analysis on Linux-next has detected a potential issue with the
> > following commit:
> >
> > commit 941168f8b40e50518a3bc6ce770a7062a5d99230
> > Author: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > Date:   Fri Apr 26 11:39:24 2024 +0800
> >
> >     virtio_net: support device stats
> >
> >
> > The issue is in function virtnet_stats_ctx_init, in drivers/net/virtio_net.c
> > as follows:
> >
> >         if (vi->device_stats_cap & VIRTIO_NET_STATS_TYPE_CVQ) {
> >                 queue_type = VIRTNET_Q_TYPE_CQ;
> >
> >                 ctx->bitmap[queue_type]   |= VIRTIO_NET_STATS_TYPE_CVQ;
> >                 ctx->desc_num[queue_type] +=
> > ARRAY_SIZE(virtnet_stats_cvq_desc);
> >                 ctx->size[queue_type]     += sizeof(struct
> > virtio_net_stats_cvq);
> >         }
> >
> >
> > ctx->bitmap is declared as a u32 however it is being bit-wise or'd with
> > VIRTIO_NET_STATS_TYPE_CVQ and this is defined as 1 << 32:
> >
> > include/uapi/linux/virtio_net.h:#define VIRTIO_NET_STATS_TYPE_CVQ (1ULL <<
> > 32)
> >
> > ..and hence the bit-wise or operation won't set any bits in ctx->bitmap
> > because 1ULL < 32 is too wide for a u32.
>
> Indeed. Xuan Zhuo how did you test this patch?

In our machines, cvq type is not supported. ^_^

Why gcc has not warning for this, or I missed it.

>
> > I suspect ctx->bitmap should be
> > declared as u64.
> >
> > Colin
> >
> >
>
> In fact, it is read into a u64:
>
>        u64 offset, bitmap;
> ....
>         bitmap = ctx->bitmap[queue_type];
>
> we'll have to reorder fields to avoid wasting memory.
> Like this I guess:
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>

Thanks

>
> Colin, can you confirm pls?
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c6af18948092..ef221429f784 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -4111,12 +4111,12 @@ struct virtnet_stats_ctx {
>  	/* Used to calculate the offset inside the output buffer. */
>  	u32 desc_num[3];
>
> -	/* The actual supported stat types. */
> -	u32 bitmap[3];
> -
>  	/* Used to calculate the reply buffer size. */
>  	u32 size[3];
>
> +	/* The actual supported stat types. */
> +	u64 bitmap[3];
> +
>  	/* Record the output buffer. */
>  	u64 *data;
>  };
> --
> MST
>