Re: [PATCH 00/12] ACPI/NVDIMM: Runtime Firmware Activation

[Dan Williams <dan.j.williams@intel.com>]

On Sun, Jun 28, 2020 at 10:23 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Fri, Jun 26, 2020 at 8:43 PM Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > On Fri, Jun 26, 2020 at 7:22 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > >
> > > On Fri, Jun 26, 2020 at 2:06 AM Dan Williams <dan.j.williams@intel.com> wrote:
> > > >
> > > > Quoting the documentation:
> > > >
> > > >     Some persistent memory devices run a firmware locally on the device /
> > > >     "DIMM" to perform tasks like media management, capacity provisioning,
> > > >     and health monitoring. The process of updating that firmware typically
> > > >     involves a reboot because it has implications for in-flight memory
> > > >     transactions. However, reboots are disruptive and at least the Intel
> > > >     persistent memory platform implementation, described by the Intel ACPI
> > > >     DSM specification [1], has added support for activating firmware at
> > > >     runtime.
> > > >
> > > >     [1]: https://docs.pmem.io/persistent-memory/
> > > >
> > > > The approach taken is to abstract the Intel platform specific mechanism
> > > > behind a libnvdimm-generic sysfs interface. The interface could support
> > > > runtime-firmware-activation on another architecture without need to
> > > > change userspace tooling.
> > > >
> > > > The ACPI NFIT implementation involves a set of device-specific-methods
> > > > (DSMs) to 'arm' individual devices for activation and bus-level
> > > > 'trigger' method to execute the activation. Informational / enumeration
> > > > methods are also provided at the bus and device level.
> > > >
> > > > One complicating aspect of the memory device firmware activation is that
> > > > the memory controller may need to be quiesced, no memory cycles, during
> > > > the activation. While the platform has mechanisms to support holding off
> > > > in-flight DMA during the activation, the device response to that delay
> > > > is potentially undefined. The platform may reject a runtime firmware
> > > > update if, for example a PCI-E device does not support its completion
> > > > timeout value being increased to meet the activation time. Outside of
> > > > device timeouts the quiesce period may also violate application
> > > > timeouts.
> > > >
> > > > Given the above device and application timeout considerations the
> > > > implementation defaults to hooking into the suspend path to trigger the
> > > > activation, i.e. that a suspend-resume cycle (at least up to the syscore
> > > > suspend point) is required.
> > >
> > > Well, that doesn't work if the suspend method for the system is set to
> > > suspend-to-idle (for example, via /sys/power/mem_sleep), because the
> > > syscore callbacks are not invoked in that case.
> > >
> > > Also you probably don't need the device power state toggling that
> > > happens during regular suspend/resume (you may not want it even for
> > > some devices).
> > >
> > > The hibernation freeze/thaw may be a better match and there is some
> > > test support in there already that may be kind of co-opted for your
> > > use case.
> >
> > Hmm, yes I guess freeze should be sufficient to quiesce most
> > device-DMA in the general case as applications will stop sending
> > requests.
>
> It is expected to be sufficient to quiesce all of them.
>
> If that is not the case, the integrity of the hibernation image cannot
> be guaranteed on the system in question.
>

Ah, indeed, I was overlooking that property.

> > I do expect some RDMA devices will happily keep on
> > transmitting, but that likely will need explicit mitigation. It also
> > appears the suspend callback for at least one RDMA device
> > mlx5_suspend() is rather violent as it appears to fully teardown the
> > device context, not just suspend operations.
> >
> > To be clear, what debug interface were you thinking I could glom onto
> > to just trigger firmware-activate at the end of the freeze phase?
>
> Functionally, the same as for suspend, but using the hibernation
> interface, so "echo platform > /sys/power/pm_test" followed by "echo
> disk > /sys/power/state".
>
> But it might be cleaner to introduce a special "hibernation mode", ie.
> is one more item in /sys/power/disk, that will trigger what you need
> (in analogy with "test_resume").

I'll move the trigger to be after process freeze, but I'll keep it
tied to suspend-debug vs hibernate-debug. It appears the hibernate
debug path still goes through the exercise of allocating memory for
the hibernation image which is unnecessary if the goal is just to
'freeze', 'activate', and 'thaw'.
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