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Date:   Fri, 26 Mar 2021 08:20:07 -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 Fri, 26 Mar 2021 09:40:30 +0300
Leon Romanovsky <leon@kernel.org> wrote:

> On Thu, Mar 25, 2021 at 11:53:24AM -0600, Alex Williamson wrote:
> > 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?    

(3) This +/- scheme, which doesn't support re-ordering.

> > > 
> > > 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.  

(2) This, which I believe is in reference to... ^^
> > 
> > The above doesn't support re-ordering though, we'll need to parse a
> > comma separated list for that.

(1) This refers to... ^^
  
> 
> It supports by writing: echo "bus,pm" > reset_methods.
> Regarding comma, IMHO it is easiest pattern for the parsing.
> 
> Anyway, The in-kernel implementation is not important to me.

Too bad, it should have been apparent from the sample code that it was
using a comma separated list with re-ordering support.  Thanks,

Alex