Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

From: Rusty Russell
Date: Sun Jan 06 2013 - 20:17:46 EST

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> Il 02/01/2013 06:03, Rusty Russell ha scritto:
>> Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
>>> 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

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.

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