Re: [PATCH v3] PCI/MSI: Don't touch MSI bits when the PCI device is disconnected

[Oliver <oohall@gmail.com>]

On Thu, Mar 21, 2019 at 12:27 PM Alex G <mr.nuke.me@gmail.com> wrote:
>
> On 3/20/19 4:44 PM, Linus Torvalds wrote:
> > On Wed, Mar 20, 2019 at 1:52 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> >>
> >> AFAICT, the consensus there was that it would be better to find some
> >> sort of platform solution instead of dealing with it in individual
> >> drivers.  The PCI core isn't really a driver, but I think the same
> >> argument applies to it: if we had a better way to recover from readl()
> >> errors, that way would work equally well in nvme-pci and the PCI core.
> >
> > I think that patches with the pattern "if (disconnected) don't do IO"
> > are fundamentally broken and we should look for alternatives in all
> > cases.
> >
> > They are fundamentally broken because they are racy: if it's an actual
> > sudden disconnect in the middle of IO, there's no guarantee that we'll
> > even be notified in time.
> >
> > They are fundamentally broken because they add new magic special cases
> > that very few people will ever test, and the people who do test them
> > tend to do so with old irrelevant kernels.
> >
> > Finally, they are fundamentally broken because they always end up
> > being just special cases. One or two special case accesses in a
> > driver, or perhaps all accesses of a particular type in just _one_
> > special driver.
> >
> > Yes, yes, I realize that people want error reporting, and that
> > hot-removal can cause various error conditions (perhaps just parity
> > errors for the IO, but also perhaps other random errors caused by
> > firmware perhaps doing special HW setup).
> >
> > But the "you get a fatal interrupt, so avoid the IO" kind of model is
> > completely broken, and needs to just be fixed differently. See above
> > why it's so completely broken.
> >
> > So if the hw is set up to send some kinf of synchronous interrupt or
> > machine check that cannot sanely be handled (perhaps because it will
> > just repeat forever), we should try to just disable said thing.
> >
> > PCIe allows for just polling for errors on the bridges, afaik. It's
> > been years since I looked at it, and maybe I'm wrong. And I bet there
> > are various "platform-specific value add" registers etc that may need
> > tweaking outside of any standard spec for PCIe error reporting. But
> > let's do that in a platform driver, to set up the platform to not do
> > the silly "I'm just going to die if I see an error" thing.
> >
> > It's way better to have a model where you poll each bridge once a
> > minute (or one an hour) and let people know "guys, your hardware
> > reports errors", than make random crappy changes to random drivers
> > because the hardware was set up to die on said errors.
> >
> > And if some MIS person wants the "hardware will die" setting, then
> > they can damn well have that, and then it's not out problem, but it
> > also means that we don't start changing random drivers for that insane
> > setting. It's dead, Jim, and it was the users choice.
> >
> > Notice how in neither case does it make sense to try to do some "if
> > (disconnected) dont_do_io()" model for the drivers.
>
> I disagree with the idea of doing something you know can cause an error
> to propagate. That being said, in this particular case we have come to
> rely a little too much on the if (disconnected) model.

My main gripe with the if (disconnected) model is that it's only
really good for inactive devices. If a device is being used then odds
are the driver will do an MMIO before the pci core has had a chance to
mark the device as broken so you crash anyway.

> You mentioned in the other thread that fixing the GHES driver will pay
> much higher dividends. I'm working on reviving a couple of changes to do
> just that. Some industry folk were very concerned about the "don't
> panic()" approach, and I want to make sure I fairly present their
> arguments in the cover letter.
>
> I'm hoping one day we'll have the ability to use page tables to prevent
> the situations that we're trying to fix today in less than ideal ways.
> Although there are other places in msi.c that use if (disconnected), I'm
> okay with dropping this change -- provided we come up with an equivalent
> fix.

What's the idea there? Scan the ioremap space for mappings over the
device BARs and swap them with a normal memory page?

> But even if FFS doesn't crash, do we really want to lose hundreds of
> milliseconds to SMM --on all cores-- when all it takes is a couple of
> cycles to check a flag?

Using pci_dev_is_disconnected() to opportunistically avoid waiting for
MMIO timeouts is fair enough IMO, even if it's a bit ugly. It would
help your case if you did some measurements to show the improvement
and look for other cases it might help. It might also be a good idea
to document when it is appropriate to use pci_is_dev_disconnected() so
we aren't stuck having the same argument again and again, but that's
probably a job for Bjorn though.

Oliver