On Fri, 26 Mar 2021 09:40:30 +0300 Leon Romanovsky <l...@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 <l...@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 <l...@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 <l...@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
