Messages in this thread |  | | From | Rusty Russell <> | Subject | Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers | Date | Mon, 07 Jan 2013 10:32:19 +1030 |
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Paolo Bonzini <pbonzini@redhat.com> writes: > Il 02/01/2013 06:03, Rusty Russell ha scritto: >> Paolo Bonzini <pbonzini@redhat.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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