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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

>> 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