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

From: Alex G
Date: Wed Mar 20 2019 - 21:27:32 EST

On 3/20/19 4:44 PM, Linus Torvalds wrote:
On Wed, Mar 20, 2019 at 1:52 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> 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

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.

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.

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?