Re: [PATCH 1/9] Xen Share: Simplified I/O Mechanism

["Mike D. Day" <ncmike@us.ibm.com>]

Rusty Russell wrote:
> Finally, a scatter-gather list mechanism allows the domains to
> associate their pages with arbitrary queue numbers in the shared
> region, to transport bulk data (effectively by having the hypervisor
> do "DMA" between domains).

The sg transfer mechanism avoids bounce buffers and does a straight copy 
  from_user or to_user below. So this is an alternative to page 
flipping, right? It would be interesting to see how the direct copy 
performs compared to page flipping on various architectures. Page table 
updates are not free and processors are getting more efficient at bulk dma.

> > +/* Copy from src to dst, return amount copied. */
> +static int do_copy(const struct sg_list *sgdst, const struct sg_list *sgsrc,
> +		   unsigned long (*copy)(paddr_t, paddr_t, unsigned long))
> +{
> +	unsigned long totlen, src, dst, srcoff, dstoff;
> +	int ret = 0;
> +
> +	totlen = 0;
> +	src = dst = 0;
> +	srcoff = dstoff = 0;
> +	while (src < sgsrc->num_sg) {
> +		unsigned long len;
> +		len = min(sgsrc->sg[src].len - srcoff,
> +			  sgdst->sg[dst].len - dstoff);
> +
> +		len = copy(sgdst->sg[dst].addr+dstoff,
> +			   sgsrc->sg[src].addr+srcoff,
> +			   len);