Messages in this thread | | | Date | Fri, 08 Feb 2013 07:35:44 +0100 | From | Paolo Bonzini <> | 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@redhat.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
| |