Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper function

[Logan Gunthorpe]

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

Yes, sort of...

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.

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.

But, yes, if performance is not an issue, switching it to
sg_copy_to_buffer would be a less invasive change than sg_miter. Which
the same might be said about a lot of these cases.

Unfortunately, changing from kmap_atomic (which is a null operation in a
lot of cases) to sg_copy_X_buffer is a pretty big performance hit.

Logan