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

From: Rusty Russell
Date: Sun Jan 06 2013 - 18:42:35 EST

Wanlong Gao <gaowanlong@xxxxxxxxxxxxxx> writes:
> On 01/02/2013 01:03 PM, Rusty Russell wrote:
>> 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.
> I rebased against virtio-next and use it in virtio-scsi, and tested it with 4 targets
> virtio-scsi devices and host cpu idle=poll. Saw a little performance regression here.

Sure, but now you should be able to eliminate virtscsi_map_sgl(), right?
You should be able to use scsi_out(sc) and scsi_in(sc) directly, which
is what Paulo wanted to do...

Right Paulo?

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