On Thu, Oct 18, 2018 at 08:06:05AM -0700, Matthew Wilcox wrote:

Can you name one that does require 512-byte alignment, preferably still
in use?  Or even >4-byte alignment.  I just checked AHCI and that requires
only 2-byte alignment.

Xen-blkfront, rsxx, various SD/MMC card readers for example.

I have reason to believe that these are uncommon because of the feedback
we got in the NVMe committee after releasing 1.0 which required 4-byte
alignment from people whining that they just couldn't guarantee 4-byte
alignment in their host devices and they absolutely needed to have no
alignment requirements (!)

See how things turned - after NVMe SGLs followed the no alignment
rule enough controller vendors rebelled so that NVMe 1.3 has an option
of SGL support only if 4-byte aligned.