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

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Date:	Mon, 07 Jan 2013 10:32:19 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	kvm@...r.kernel.org, mst@...hat.com, hutao@...fujitsu.com,
	virtualization@...ts.linux-foundation.org, stefanha@...hat.com,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

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