Re: [PATCH v2 1/5] virtio: add functions for piecewise addition ofbuffers
From: Paolo Bonzini
Date: Mon Jan 07 2013 - 09:27:44 EST
Il 07/01/2013 01:02, Rusty Russell ha scritto:
> 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?
It needs to use the iterator functions in its DMA driver.
> All archs we care about support them, though, so I think we can ignore
> this issue for now.
Kind of... In principle all QEMU-supported arches can use virtio, and
the speedup can be quite useful. And there is no Kconfig symbol for SG
chains that I can use to disable virtio-scsi on unsupported arches. :/
>> (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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