On Thu, Apr 27, 2017 at 03:53:37PM -0600, Logan Gunthorpe wrote:

On 27/04/17 02:53 PM, Jason Gunthorpe wrote:

quoted

blkfront is one of the drivers I looked at, and it appears to only be
memcpying with the bvec_data pointer, so I wonder why it does not use
sg_copy_X_buffer instead..

But you'd potentially end up calling sg_copy_to_buffer multiple times
per page within the sg (given that gnttab_foreach_grant_in_range might
call blkif_copy_from_grant/blkif_setup_rw_req_grant multiple times).
Even calling sg_copy_to_buffer once per page seems rather inefficient as
it uses sg_miter internally.

Well, that is in the current form, with more users it would make sense
to optimize for the single page case, eg by providing the existing
call, providing a faster single-page-only variant of the copy, perhaps
even one that is inlined.

Switching the for_each_sg to sg_miter is probably the nicer solution as
it takes care of the mapping and the offset/length accounting for you
and will have similar performance.

sg_miter will still fail when the sg contains __iomem, however I would
expect that the sg_copy will work with iomem, by using the __iomem
memcpy variant.

So, sg_copy should always be preferred in this new world with mixed
__iomem since it is the only primitive that can transparently handle
it.

Jason