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

[Rafael J. Wysocki]

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.

> 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").