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

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Message-ID: <51149CC0.4080702@redhat.com>
Date:	Fri, 08 Feb 2013 07:35:44 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	kvm@...r.kernel.org, mst@...hat.com,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist
 changes

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

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.

Paolo
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