Re: [PATCH v2 11/12] PM, libnvdimm: Add 'mem-quiet' state and callback for firmware activation

[Rafael J. Wysocki]

On Thursday, July 9, 2020 9:04:30 PM CEST Dan Williams wrote:
> On Thu, Jul 9, 2020 at 7:57 AM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> >
> > On Tuesday, July 7, 2020 3:59:32 AM CEST Dan Williams wrote:
> > > The runtime firmware activation capability of Intel NVDIMM devices
> > > requires memory transactions to be disabled for 100s of microseconds.
> > > This timeout is large enough to cause in-flight DMA to fail and other
> > > application detectable timeouts. Arrange for firmware activation to be
> > > executed while the system is "quiesced", all processes and device-DMA
> > > frozen.
> > >
> > > It is already required that invoking device ->freeze() callbacks is
> > > sufficient to cease DMA. A device that continues memory writes outside
> > > of user-direction violates expectations of the PM core to be to
> > > establish a coherent hibernation image.
> > >
> > > That said, RDMA devices are an example of a device that access memory
> > > outside of user process direction. RDMA drivers also typically assume
> > > the system they are operating in will never be hibernated. A solution
> > > for RDMA collisions with firmware activation is outside the scope of
> > > this change and may need to rely on being able to survive the platform
> > > imposed memory controller quiesce period.
> >
> > Thanks for following my suggestion to use the hibernation infrastructure
> > rather than the suspend one, but I think it would be better to go a bit
> > further with that.
> >
> > Namely, after thinking about this a bit more I have come to the conclusion
> > that what is needed is an ability to execute a function, inside of the
> > kernel, in a "quiet" environment in which memory updates are unlikely.
> >
> > While the hibernation infrastructure as is can be used for that, kind of, IMO
> > it would be cleaner to introduce a helper for that, like in the (untested)
> > patch below, so if the "quiet execution environment" is needed, whoever
> > needs it may simply pass a function to hibernate_quiet_exec() and provide
> > whatever user-space I/F is suitable on top of that.
> >
> > Please let me know what you think.
> 
> This looks good to me in concept.
> 
> Would you expect that I trigger this from libnvdimm sysfs, or any
> future users of this functionality to trigger it through their own
> subsystem specific mechanisms?

Yes, I would.

> I have a place for it in libvdimm and could specify the activation
> method directly as "suspend" vs "live" activation.

Sounds good to me.

Cheers!