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

[Christoph Hellwig]

On Mon, Jul 06, 2020 at 06:59:32PM -0700, 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.

Yikes.  I don't think we should support such a broken runtime firmware
activation.