shadow doorbell on admin queue

[Klaus Jensen]

Hi,

Jinhao, our QEMU GSoC intern is working on nvme performance improvements
and while implementing shadow doorbells an interesting issue came up.

The spec states that when enabling shadow doorbells, it is for all
queues (including the admin queue). However, the kernel will currently
not update the shadow doorbell for the admin queue, which causes trouble
if the device expects it to.

The kernel is not the only driver doing this - SPDK doesn't update it
either[1]. At least one virtual target implementing shadow doorbells
(SPDK's vfio-user target) jumped on the band-wagon and simply expects
the driver to not update it. In QEMU, Keith came up with a hack to
update the shadow doorbell from the device side, allowing both compliant
and non-compliant drivers to work. Just fixing it on the driver side is
a problem, because it will break targets that expects the non-compliant
behavior (i.e. always expecting mmio on the admin queue).

Question is if the kernel even wants to do anything about this at all? I
kinda already know the answer here - "spec is screwed up", but I wanted
to raise the issue here for feedback prior to potentially starting a
process with the NVMe TWG to sort out there.


  [1]: https://github.com/spdk/spdk/issues/2147
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