[PATCH v8 24/24] nvme-pci: optimize single-segment handling

[Christoph Hellwig]

On Fri, Apr 18, 2025 at 09:47:54AM +0300, Leon Romanovsky wrote:
> From: Kanchan Joshi <joshi.k at samsung.com>
> 
> blk_rq_dma_map API is costly for single-segment requests.
> Avoid using it and map the bio_vec directly.

This needs to be folded into the earlier patches or split prep patches
instead of undoing work done earlier, preferably combined with a bit
of code movement so that the new nvme_try_setup_prp_simple stays in
the same place as before and the diff shows it reusing code.

E.g. change

"nvme-pci: use a better encoding for small prp pool allocations" to
already use the flags instead of my boolean, and maybe include 
abort in the flags instead of using a separate bool so that we
don't increase hte iod size.

Slot in a new patch after that that dropping the single SGL segment
fastpath if we think we don't need that, although if we need the PRP
one I suspect that one would still be very helpful as well.

Add a patch if we want the try_ version of, although when keeping
the optimization for SGLs as well that are will look a bit different.

I'm happy to give away my patch authorship credits if that helps with
the folding.