SGL support of driver

[Keith Busch]

On Mon, 27 Jul 2015, Christoph Hellwig wrote:
> On Mon, Jul 27, 2015 at 09:22:51AM -0700, sheng qiu wrote:
>> Hi,
>>
>> may i ask if exiting kernel driver support SGL DMA setups?
>
> It doesn't.  There have been patches but they weren't in shape to be
> included yet.

We also need data that shows SGL is an improvement over PRP. H/w should
be able to process a PRP faster than an SGL in most cases.

SGL may be faster with a physically contiguous range across multiple
pages, or if a request payload can't be PRP mapped, requiring a bounce
buffer.

The Linux driver never recieves an IO vector that requires double
buffering though, and usually not physically contiguous. I'm not sure
there is a case to support SGL in the linux-nvme driver. Maybe if it's
tied to block integrity extensions with interleaved metadata formats,
but it'd be odd to find a device that implements SGL but not separate
metadata.