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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 9 Jul 2020 12:04:30 -0700
Message-ID: <CAPcyv4iiYMXO1fH0yQ2eBzpOWqPag0W=ebJwV6spGpNJQ9hnrg@mail.gmail.com>
Subject: Re: [PATCH v2 11/12] PM, libnvdimm: Add 'mem-quiet' state and
 callback for firmware activation
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     linux-nvdimm <linux-nvdimm@lists.01.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Ira Weiny <ira.weiny@intel.com>, Pavel Machek <pavel@ucw.cz>,
        Len Brown <len.brown@intel.com>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Jul 9, 2020 at 7:57 AM Rafael J. Wysocki <rjw@rjwysocki.net> 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?

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