Re: Is there any plan to support CXL GPF in Linux

[Dan Williams <dan.j.williams@intel.com>]

Hi Yee, please refrain from top-posting [1]

[1]: https://subspace.kernel.org/etiquette.html

Yee Li wrote:
> Dear Christoph,
> 
> I agree with you.
> cxl pmem device should process the GPF event well by itself which is sent
> from Host or Switch.
> 
> Dear Dan,
> 
> ACPI.FADT.PERSISTENT_CPU_CACHES flag is not recognized by Linux.
> Does it mean "cxl pmem flush operations reference the NFIT Platform
> Capabilities"? Or, support the flag and cap in the future.

The Persistent CPU Caches flag is a performance optimization to avoid
CPU cache flushing when the platform is trusted to take care of cache
flushing at power-loss.

Software can force "persistent caches" behavior by:

    echo 0 > /sys/block/pmem0/dax/write_cache

From a kernel developer perspective, I feel more comfortable with
userspace making that "go fast at the risk of data-loss" decision. Maybe
after the industry gets more experience with platforms that set that bit
the kernel can default to trusting it, but given the slow roll out of
CXL PMEM devices I think Linux is ok to take a "wait and see" attitude.
At a minimum, if someone wants to draft a patch, it should be a build
time configuration option to trust the FADT.