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Date:   Thu, 25 Mar 2021 11:53:24 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Leon Romanovsky <leon@kernel.org>
Cc:     "Enrico Weigelt, metux IT consult" <info@metux.net>,
        Amey Narkhede <ameynarkhede03@gmail.com>,
        raphael.norwitz@nutanix.com, linux-pci@vger.kernel.org,
        bhelgaas@google.com, linux-kernel@vger.kernel.org,
        alay.shah@nutanix.com, suresh.gumpula@nutanix.com,
        shyam.rajendran@nutanix.com, felipe@nutanix.com
Subject: Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device
 reset mechanism
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On Thu, 25 Mar 2021 18:09:58 +0200
Leon Romanovsky <leon@kernel.org> wrote:

> On Thu, Mar 25, 2021 at 08:55:04AM -0600, Alex Williamson wrote:
> > On Thu, 25 Mar 2021 10:37:54 +0200
> > Leon Romanovsky <leon@kernel.org> wrote:
> >   
> > > On Wed, Mar 24, 2021 at 11:17:29AM -0600, Alex Williamson wrote:  
> > > > On Wed, 24 Mar 2021 17:13:56 +0200
> > > > Leon Romanovsky <leon@kernel.org> wrote:    
> > > 
> > > <...>
> > >   
> > > > > Yes, and real testing/debugging almost always requires kernel rebuild.
> > > > > Everything else is waste of time.    
> > > > 
> > > > Sorry, this is nonsense.  Allowing users to debug issues without a full
> > > > kernel rebuild is a good thing.    
> > > 
> > > It is far from debug, this interface doesn't give you any answers why
> > > the reset didn't work, it just helps you to find the one that works.
> > > 
> > > Unless you believe that this information will be enough to understand
> > > the root cause, you will need to ask from the user to perform extra
> > > tests, maybe try some quirk. All of that requires from the users to
> > > rebuild their kernel.
> > > 
> > > So no, it is not debug.  
> > 
> > It allows a user to experiment to determine (a) my device doesn't work
> > in a given scenario with the default configuration, but (b) if I change
> > the reset to this other thing it does work.  That is a step in
> > debugging.
> > 
> > It's absurd to think that a sysfs attribute could provide root cause,
> > but it might be enough for someone to further help that user.  It would
> > be a useful clue for a bug report.  Yes, reaching root cause might
> > involve building a kernel, but that doesn't invalidate that having a
> > step towards debugging in the base kernel might be a useful tool.  
> 
> Let's agree to do not agree.
> 
> >   
> > > > > > > > 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.        
> > > > > > > 
> > > > > > > This interface is proposed as first class citizen in the general sysfs
> > > > > > > layout. Of course, it will be seen as a way to bypass the kernel.
> > > > > > > 
> > > > > > > At least, put it under CONFIG_EXPERT option, so no distro will enable it
> > > > > > > by default.      
> > > > > > 
> > > > > > Of course we're proposing it to be accessible, it should also require
> > > > > > admin privileges to modify, sysfs has lots of such things.  If it's
> > > > > > relegated to non-default accessibility, it won't be used for testing
> > > > > > and it won't be available for system policy and it's pointless.      
> > > > > 
> > > > > We probably have difference in view of what testing is. I expect from
> > > > > the users who experience issues with reset to do extra steps and one of
> > > > > them is to require from them to compile their kernel.    
> > > > 
> > > > I would define the ability to generate a CI test that can pick a
> > > > device, unbind it from its driver, and iterate reset methods as a
> > > > worthwhile improvement in testing.    
> > > 
> > > Who is going to run this CI? At least all kernel CIs (external and
> > > internal to HW vendors) that I'm familiar are building kernel themselves.
> > > 
> > > Distro kernel is too bloat to be really usable for CI.  
> > 
> > At this point I'm suspicious you're trolling.  A distro kernel CI
> > certainly uses the kernel they intend to ship and support in their
> > environment. You're concerned about a bloated kernel, but the proposal
> > here adds 2-bytes per device to track reset methods and a trivial array
> > in text memory, meanwhile you're proposing multiple per-device memory
> > allocations to enhance the feature you think is too bloated for CI.  
> 
> I don't know why you decided to focus on memory footprint which is not
> important at all during CI runs. The bloat is in Kconfig options that
> are not needed. Those extra options add significant overhead during
> builds and runs itself.
> 
> And not, I'm not trolling, but representing HW vendor that pushes its CI
> and developers environment to the limit, by running full kernel builds with
> less than 30 seconds and boot-to-test with less than 6 seconds for full
> Fedora VM.

CI is only one aspect where I think this interface could be useful, as
below there's also a policy use case.  Therefore my inclination is that
this would be included in default kernels and avoiding bloat is a good
thing.  A CI environment can be used in different ways, it's not
necessarily building a new kernel for every test, nor do typical users
have access to those types of environments to report information in a
bug.
   
> > > > > The root permissions doesn't protect from anything, SO lovers will use
> > > > > root without even thinking twice.    
> > > > 
> > > > Yes, with great power comes great responsibility.  Many admins ignore
> > > > this.  That's far beyond the scope of this series.    
> > > 
> > > <...>
> > >   
> > > > > I'm trying to help you with your use case of providing reset policy
> > > > > mechanism, which can be without CONFIG_EXPERT. However if you want
> > > > > to continue path of having specific reset type only, please ensure
> > > > > that this is not taken to the "bypass kernel" direction.    
> > > > 
> > > > You've lost me, are you saying you'd be in favor of an interface that
> > > > allows an admin to specify an arbitrary list of reset methods because
> > > > that's somehow more in line with a policy choice than a userspace
> > > > workaround?  This seems like unnecessary bloat because (a) it allows
> > > > the same bypass mechanism, and (b) a given device is only going to use
> > > > a single method anyway, so the functionality is unnecessary.  Please
> > > > help me understand how this favors the policy use case.  Thanks,    
> > > 
> > > The policy decision is global logic that is easier to grasp. At some
> > > point of our discussion, you presented the case where PM reset is not
> > > defined well and you prefer to do bus reset (something like that).
> > > 
> > > I expect that QEMU sets same reset policy for all devices at the same
> > > time instead of trying per-device to guess which one works.
> > > 
> > > And yes, you will be able to bypass kernel, but at least this interface
> > > will be broader than initial one that serves only SO and workarounds.  
> > 
> > I still think allocating objects for a list and managing that list is
> > too bloated and complicated, but I agree that being able to have more
> > fine grained control could be useful.  Is it necessary to be able to
> > re-order reset methods or might it still be better aligned to a policy
> > use case if we allow plus and minus operators?  For example, a device
> > might list:
> > 
> > [pm] [bus]
> > 
> > Indicating that PM and bus reset are both available and enabled.  The
> > user could do:
> > 
> > echo -pm > reset_methods
> > 
> > This would result in:
> > 
> > pm [bus]
> > 
> > Indicating that both PM and bus resets are available, but only bus reset
> > is enabled (note this is the identical result to "echo bus >" in the
> > current proposal).  "echo +pm" or "echo default" could re-enable the PM
> > reset.  Would something like that be satisfactory?  
> 
> Yes, I actually imagined simpler interface:
> To set specific type:
> echo pm > reset_methods
> To set policy:
> echo "pm,bus" > reset_methods
> 
> But your proposal is nicer.

The above doesn't support re-ordering though, we'll need to parse a
comma separated list for that.

> > If we need to allow re-ording, we'd want to use a byte-array where each
> > byte indicates a type of reset and perhaps a non-zero value in the
> > array indicates the method is enabled and the value indicates priority.
> > For example writing "dev_spec,flr,bus" would parse to write 1 to the
> > byte associated with the device specific reset, 2 to flr, 3 to bus
> > reset, then we'd process low to high (or maybe starting at a high value
> > to count down to zero might be more simple).  We could do that with
> > only adding less than a fixed 8-bytes per device and no dynamic
> > allocation.  Thoughts?  Thanks,  
> 
> Like I suggested, linked list will be easier and the reset will be
> something like:
>  for_each_reset_type(device, type) { 
>    switch (type) {
>    	case PM:
> 	       ret = do_some_reset(device);
> 	       break;
> 	case BUS:
> 		.....
> 	}
>    if (!ret || ret == -ENOMEM)  <-- go to next type in linked list
>      return ret;
>    }

Perhaps Bjorn has some thoughts, but I don't like the dynamic memory
allocation and list management required for a linked list.  Once bus &
slot reset are combined, I think we're talking about potentially 5
reset methods, so if we had:

const struct pci_reset_fn_method pci_reset_fn_methods[] = {
	{ .reset_fn = &pci_dev_specific_reset, .name = "device_specific" },
	{ .reset_fn = &pcie_flr, .name = "flr" },
	{ .reset_fn = &pci_af_flr, .name = "af_flr" },
	{ .reset_fn = &pci_pm_reset, .name = "pm" },
	{ .reset_fn = &pci_reset_bus_function, .name = "bus" },
};

The pci_dev could include

	u8 reset_methods[ARRAY_SIZE(pci_reset_fn_methods)];

And we could loop as:

u8 prio;

for (prio = ARRAY_SIZE(pci_reset_fn_methods); prio; prio--) {
	int i;

	for (i = 0; i < ARRAY_SIZE(pci_reset_fn_methods); i++) {
		if (dev->reset_methods[i] == prio) {
			ret = pci_reset_fn_methods[i].reset_fn(dev, probe);
			if (ret != -ENOTTY)
				return ret;
			break;
		}
	}
	if (i == ARRAY_SIZE(pci_reset_fn_methods))
		break;
}

return -ENOTTY;

The sysfs _store function would probably do something like:

u8 reset_methods[ARRAY_SIZE(pci_reset_fn_methods)] = { 0 };
u8 prio = ARRAY_SIZE(pci_reset_fn_methods);

for each @string in comma separated list from user... {
	int i;

	for (i = 0; i < ARRAY_SIZE(pci_reset_fn_methods); i++) {
		if (!strcmp(@string, pci_reset_fn_methods[i].name)) {
			reset_methods[i] = prio--;
			break;
		}
	}

	if (i == ARRAY_SIZE(pci_reset_fn_methods))
		return -EINVAL;
}

memcpy(dev->reset_methods, reset_methods, sizeof(reset_methods));

The probe would also need to fill the array in a compatible way:

u8 reset_methods[ARRAY_SIZE(pci_reset_fn_methods)] = { 0 };
u8 prio = ARRAY_SIZE(pci_reset_fn_methods);
int i;

for (i = 0; i < ARRAY_SIZE(pci_reset_fn_methods); i++) {
	int ret = pci_reset_fn_methods[i].reset_fn(dev, 1);

	if (!ret)
		reset_methods[i] = prio--;
	else if (ret != -ENOTTY)
		break;
}
	
memcpy(dev->reset_methods, reset_methods, sizeof(reset_methods));

Thanks,
Alex