Re: [PATCH 1/2] hw/nvme: Implement shadow doorbell buffer support

[Klaus Jensen <its@irrelevant.dk>]

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On Jun 14 08:41, Keith Busch wrote:
> It's a pretty nasty hack, and definitely not in compliance with the spec: the
> db_addr is supposed to be read-only from the device side, though I do think
> it's safe for this environment. Unless Klaus or anyone finds something I'm
> missing, I feel this is an acceptable compromise to address this odd
> discrepency.
> 

No, I love this hack! :D

I have tested your hack against a dbbuf enabled driver that enables
shadow doorbells on the admin queue by default. I can confirm that this
works as well as on "broken" (or, lets call them "reasonable") drivers.

> By the way, I noticed that the patch never updates the cq's ei_addr value. Is
> that on purpose?

Yeah, I also mentioned this previously[1] and I still think we need to
update the event index. Otherwise (and my testing confirms this), we end
up in a situation where the driver skips the mmio, leaving a completion
queue entry "in use" on the device until some other completion comes
along.

I have folded these changes into a patch for testing[2]. Note, your
patch was missing equivalent changes in nvme_post_cqes(), so I added
that as well as updating of the event index.


  [1]: https://lore.kernel.org/qemu-devel/YqEMwsclktptJvQI@apples/
  [2]: http://git.infradead.org/qemu-nvme.git/commitdiff/60712930e441b684490a792b00ef6698cc85f116


Cheers,
Klaus

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