Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlistchanges

From: Paolo Bonzini
Date: Fri Feb 08 2013 - 01:35:56 EST

Il 08/02/2013 05:05, Rusty Russell ha scritto:
> 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:

Good catch. :)

> 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;
> };
> 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 they? Given the resistance you have had on the topic, perhaps they
don't (though I agree that chained scatterlist are horrible).

But I'll add a note on this to the commit message and why the workaround
is IMHO acceptable.

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