Paolo Bonzini [off-list ref] writes:

Il 02/01/2013 06:03, Rusty Russell ha scritto:

quoted

Paolo Bonzini [off-list ref] writes:

quoted

The virtqueue_add_buf function has two limitations:

1) it requires the caller to provide all the buffers in a single call;

2) it does not support chained scatterlists: the buffers must be
provided as an array of struct scatterlist;

Chained scatterlists are a horrible interface, but that doesn't mean we
shouldn't support them if there's a need.

I think I once even had a patch which passed two chained sgs, rather
than a combo sg and two length numbers.  It's very old, but I've pasted
it below.

Duplicating the implementation by having another interface is pretty
nasty; I think I'd prefer the chained scatterlists, if that's optimal
for you.

Unfortunately, that cannot work because not all architectures support
chained scatterlists.

WHAT?  I can't figure out what an arch needs to do to support this?  Why
is it an option for archs?  Why is sg_chain() even defined for
non-ARCH_HAS_SG_CHAIN?

Jens, help!!

All archs we care about support them, though, so I think we can ignore
this issue for now.

(Also, as you mention chained scatterlists are horrible.  They'd happen
to work for virtio-scsi, but not for virtio-blk where the response
status is part of the footer, not the header).

We lost that debate 5 years ago, so we hack around it as needed.  We can
add helpers to append if we need.

Thanks,
Rusty.