linux-kernel - Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes

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Message-ID: <20130208115240.GN15092@kernel.dk>
Date:	Fri, 8 Feb 2013 12:52:40 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>, asias@...hat.com,
	mst@...hat.com, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 0/8] virtio: new API for addition of buffers,
 scatterlist changes

On Fri, Feb 08 2013, Rusty Russell wrote:
> Paolo Bonzini <pbonzini@...hat.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.
> >
> > 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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