Re: [PATCH] PCI: Add Intel remapped NVMe device support

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Jun 12, 2019 at 11:16:03AM +0800, Daniel Drake wrote:
> On Wed, Jun 12, 2019 at 3:52 AM Bjorn Helgaas <helgaas@kernel.org> wrote:

> > Why do you need these to be PCI devices?
> 
> I don't have a particular preference, but was trying to explore the
> suggestions from the last round of review:
> 
> https://marc.info/?l=linux-ide&m=147923593001525&w=2
> "implementing a bridge driver like VMD"
> http://lists.infradead.org/pipermail/linux-nvme/2017-October/013325.html
> "The right way to do this would be to expose a fake PCIe root port
> that both the AHCI and NVMe driver bind to."
> 
> > It looks like the main thing
> > you get is a hook to bind the driver to.  Could you accomplish
> > something similar by doing some coordination between the ahci and nvme
> > drivers directly, without involving PCI?
> 
> That's basically what Dan Williams originally proposed, and Christoph
> Hellwig was not particularly excited by it...
> 
> Can you take a quick at the original patches and see what you think?
> https://marc.info/?l=linux-ide&m=147709611121482&w=2
> https://marc.info/?l=linux-ide&m=147709611621483&w=2
> https://marc.info/?l=linux-ide&m=147709612221484&w=2
> https://marc.info/?l=linux-ide&m=147709612721485&w=2
> https://marc.info/?l=linux-ide&m=147709613221487&w=2

I see Christoph's objections starting at
https://marc.info/?l=linux-ide&m=147711904724908&w=2
and I agree that this AHCI/NVMe melding is ugly.

But given the existence of this ugly hardware, my opinion is that
Dan's original patch series (above) is actually a nice way to deal
with it.  That's exactly the sort of thing I was proposing.

Part of Christoph's objection was the issue of how reset works, and
that objection absolutely makes sense to me.  But IMO adding a fake
PCI host bridge and fake PCI devices that really don't work because
they have read-only config space just smears the issue over
PCI/VFIO/etc in addition to AHCI and NVMe.

Bjorn