netdev - Re: [PATCH 3/3] virtio_ring: unify direct/indirect code paths.

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Date:	Wed, 3 Sep 2014 18:59:33 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	netdev <netdev@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 3/3] virtio_ring: unify direct/indirect code paths.

On Tue, Sep 2, 2014 at 9:29 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> virtqueue_add() populates the virtqueue descriptor table from the sgs
> given.  If it uses an indirect descriptor table, then it puts a single
> descriptor in the descriptor table pointing to the kmalloc'ed indirect
> table where the sg is populated.
>
> Previously vring_add_indirect() did the allocation and the simple
> linear layout.  We replace that with alloc_indirect() which allocates
> the indirect table then chains it like the normal descriptor table so
> we can reuse the core logic.
>

> +       if (vq->indirect && total_sg > 1 && vq->vq.num_free)
> +               desc = alloc_indirect(total_sg, gfp);
> +       else
> +               desc = NULL;
> +
> +       if (desc) {
> +               /* Use a single buffer which doesn't continue */
> +               vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
> +               vq->vring.desc[head].addr = virt_to_phys(desc);
> +               /* avoid kmemleak false positive (hidden by virt_to_phys) */
> +               kmemleak_ignore(desc);
> +               vq->vring.desc[head].len = total_sg * sizeof(struct vring_desc);
> +
> +               /* Set up rest to use this indirect table. */
> +               i = 0;
> +               total_sg = 1;

This is a little too magical for me.  Would it make sense to add a new
variable for this (total_root_descs or something)?

--Andy
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