On Fri, Feb 08 2013, Rusty Russell wrote:
> 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.
> >
> > Because of these limitations, virtio-scsi has to copy each request into
> > a scatterlist internal to the driver. It cannot just use the one that
> > was prepared by the upper SCSI layers.
>
> Hi Paulo,
>
> Note that you've defined your problem in terms of your solution
> here. For clarity:
>
> The problem: we want to prepend and append to a scatterlist. We can't
> append, because the chained scatterlist implementation requires
> an element to be appended to join two scatterlists together.
>
> The solution: fix scatterlists by introducing struct sg_ring:
> struct sg_ring {
> struct list_head ring;
> unsigned int nents;
> unsigned int orig_nents; /* do we want to replace sg_table? */
> struct scatterlist *sg;
> };
This would definitely be more flexible than the current chaining.
However:
> The workaround: make virtio accept multiple scatterlists for a single
> buffer.
>
> There's nothing wrong with your workaround, but if other subsystems have
> the same problem we do, perhaps we should consider a broader solution?
Do other use cases actually exist? I don't think I've come across this
requirement before, since it was introduced (6 years ago, from a cursory
look at the git logs!).
--
Jens Axboe
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