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

[Rafael J. Wysocki]

On Tue, Jun 30, 2020 at 1:37 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Sun, Jun 28, 2020 at 10:23 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> >
> > On Fri, Jun 26, 2020 at 8:43 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > >
> > > On Fri, Jun 26, 2020 at 7:22 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> > > >
> > > > On Fri, Jun 26, 2020 at 2:06 AM Dan Williams <dan.j.williams@xxxxxxxxx> 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'.

But you need the ->freeze and ->thaw callbacks to run which does not
happen at the process freeze stage.

If you add a new hibernation mode dedicated to the NVDIMM firmware
update, though, you can instrument the code to skip the memory
allocation if this mode is selected.