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:
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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