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

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Date:	Tue, 08 Jan 2013 10:42:13 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm@...r.kernel.org, linux-scsi@...r.kernel.org, mst@...hat.com,
	hutao@...fujitsu.com, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, stefanha@...hat.com,
	"Jens Axboe" <axboe@...nel.dk>
Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

Paolo Bonzini <pbonzini@...hat.com> writes:
> Il 07/01/2013 01:02, Rusty Russell ha scritto:
>> 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?
>
> It needs to use the iterator functions in its DMA driver.

But we don't care for virtio.

>> 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. :/

Well, we #error if it's not supported.  Then the lazy architectures can
fix it.

Cheers,
Rusty.
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