Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes | Date | Fri, 08 Feb 2013 14:35:55 +1030 |
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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:
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?
Cheers, Rusty.
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