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

From: Michael S. Tsirkin
Date: Tue Dec 18 2012 - 10:03:13 EST

On Tue, Dec 18, 2012 at 03:32:15PM +0100, Paolo Bonzini wrote:
> Il 18/12/2012 14:59, Michael S. Tsirkin ha scritto:
> >>> Can't we track state internally to the virtqueue? Exposing it
> >>> seems to buy us nothing since you can't call add_buf between
> >>> start and end anyway.
> >>
> >> I wanted to keep the state for these functions separate from the
> >> rest. I don't think it makes much sense to move it to struct
> >> virtqueue unless virtqueue_add_buf is converted to use the new API
> >> (doesn't make much sense, could even be a tad slower).
> >
> > Why would it be slower?
> virtqueue_add_buf could be slower if it used the new API. That's
> because of the overhead of writing and reading from struct
> virtqueue_buf, instead of using variables in registers.

Yes but we'll get rid of virtqueue_buf.

> >> On the other hand moving it there would eliminate the dependency
> >> on virtio_ring.h. Rusty, what do you think?
> >>
> >>> And idea: in practice virtio scsi seems to always call
> >>> sg_init_one, no? So how about we pass in void* or something and
> >>> avoid using sg and count? This would make it useful for -net
> >>> BTW.
> >>
> >> It also passes the scatterlist from the LLD. It calls sg_init_one
> >> for the request/response headers.
> >
> > Try adding a _single variant. You might see unrolling a loop gives
> > more of a benefit than this whole optimization.
> Makes sense, I'll try. However, note that I *do* need the
> infrastructure in this patch because virtio-scsi could never use a
> hypothetical virtqueue_add_buf_single; requests always have at least 2
> buffers for the headers.
> However I could add virtqueue_add_sg_single and use it for those
> headers.


> The I/O buffer can keep using virtqueue_add_sg.
> Paolo
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