Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device reset mechanism

From: Alex Williamson
Date: Mon Mar 22 2021 - 13:11:00 EST

On Sun, 21 Mar 2021 10:40:55 +0200
Leon Romanovsky <leon@xxxxxxxxxx> wrote:

> On Sat, Mar 20, 2021 at 08:59:42AM -0600, Alex Williamson wrote:
> > On Sat, 20 Mar 2021 11:10:08 +0200
> > Leon Romanovsky <leon@xxxxxxxxxx> wrote:
> > > On Fri, Mar 19, 2021 at 10:23:13AM -0600, Alex Williamson wrote:
> > > >
> > > > What if we taint the kernel or pci_warn() for cases where either all
> > > > the reset methods are disabled, ie. 'echo none > reset_method', or any
> > > > time a device specific method is disabled?
> > >
> > > What does it mean "none"? Does it mean nothing supported? If yes, I think that
> > > pci_warn() will be enough. At least for me, taint is usable during debug stages,
> > > probably if device doesn't crash no one will look to see /proc/sys/kernel/tainted.
> >
> > "none" as implemented in this patch, clearing the enabled function
> > reset methods.
> It is far from intuitive, the empty string will be easier to understand,
> because "none" means no reset at all.

"No reset at all" is what "none" achieves, the
pci_dev.reset_methods_enabled bitmap is cleared. We can use an empty
string, but I think we want a way to clear all enabled resets and a way
to return it to the default. I could see arguments for an empty string
serving either purpose, so this version proposed explicitly using
"none" and "default", as included in the ABI update.

> > > > I'd almost go so far as to prevent disabling a device specific reset
> > > > altogether, but for example should a device specific reset that fixes
> > > > an aspect of FLR behavior prevent using a bus reset? I'd prefer in that
> > > > case if direct FLR were disabled via a device flag introduced with the
> > > > quirk and the remaining resets can still be selected by preference.
> > >
> > > I don't know enough to discuss the PCI details, but you raised good point.
> > > This sysfs is user visible API that is presented as is from device point
> > > of view. It can be easily run into problems if PCI/core doesn't work with
> > > user's choice.
> > >
> > > >
> > > > Theoretically all the other reset methods work and are available, it's
> > > > only a policy decision which to use, right?
> > >
> > > But this patch was presented as a way to overcome situations where
> > > supported != working and user magically knows which reset type to set.
> >
> > It's not magic, the new sysfs attributes expose which resets are
> > enabled and the order that they're used, the user can simply select the
> > next one. Being able to bypass a broken reset method is a helpful side
> > effect of getting to select a preferred reset method.
> Magic in a sense that user has no idea what those resets mean, the
> expectation is that he will blindly iterate till something works.

Which ought to actually be a safe thing to do. We should have quirks to
exclude resets that are known broken but still probe as present and I'd
be perfectly fine if we issue a warning if the user disables all resets
for a given device.

> > > If you want to take this patch to be policy decision tool,
> > > it will need to accept "reset_type1,reset_type2,..." sort of input,
> > > so fallback will work natively.
> >
> > I don't see that as a requirement. We have fall-through support in the
> > kernel, but for a given device we're really only ever going to make use
> > of one of those methods. If a user knows enough about a device to have
> > a preference, I think it can be singular. That also significantly
> > simplifies the interface and supporting code. Thanks,
> I'm struggling to get requirements from this thread. You talked about
> policy decision to overtake fallback mechanism, Amey wanted to avoid
> quirks.
> Do you have an example of such devices or we are talking about
> theoretical case?

Look at any device that already has a reset quirk and the process it
took to get there. Those are more than just theoretical cases.

For policy preference, I already described how I've configured QEMU to
prefer a bus reset rather than a PM reset due to lack of specification
regarding the scope of a PM "soft reset". This interface would allow a
system policy to do that same thing.

I don't think anyone is suggesting this as a means to avoid quirks that
would resolve reset issues and create the best default general behavior.
This provides a mechanism to test various reset methods, and thereby
identify broken methods, and set a policy. Sure, that policy might be
to avoid a broken reset in the interim before it gets quirked and
there's potential for abuse there, but I think the benefits outweigh
the risks.

> And I don't see why simple line parser with loop iterator over strchr()
> suddenly becomes complicated code.

Setting multiple bits in a bitmap is easy. How do you then go on to
allow the user to specify an ordering preference? If you have an
algorithm you'd like to propose that allows the user to manage the
ordering when enabling multiple methods without substantially
increasing the complexity, please share. IMO, a given device will
generally use one reset method and it seems sufficient to restrict user
preference to achieve all the use cases I've noted. Thanks,