Re: [PATCH v4 6/9] cxl/region: Add register_dax flag to defer DAX setup

Koralahalli Channabasappa, Smita
2025-12-12T20:00:06Z

On 12/3/2025 4:22 PM, dan.j.williams@intel.com wrote:
> Smita Koralahalli wrote:
>> Stop creating cxl_dax during cxl_region_probe(). Early DAX registration
>> can online memory before ownership of Soft Reserved ranges is finalized.
>> This makes it difficult to tear down regions later when HMEM determines
>> that a region should not claim that range.
>>
>> Introduce a register_dax flag in struct cxl_region_params and gate DAX
>> registration on this flag. Leave probe time registration disabled for
>> regions discovered during early CXL enumeration; set the flag only for
>> regions created dynamically at runtime to preserve existing behaviour.
>>
>> This patch prepares the region code for later changes where cxl_dax
>> setup occurs from the HMEM path only after ownership arbitration
>> completes.
>
> This seems backwards to me. The dax subsystem knows when it wants to
> move ahead with CXL or not, dax_cxl_mode is that indicator. So, just
> share that variable with drivers/dax/cxl.c, arrange for
> cxl_dax_region_probe() to fail while waiting for initial CXL probing to
> succeed.
>
> Once that point is reached move dax_cxl_mode to DAX_CXL_MODE_DROP, which
> means drop the hmem alias, and go with the real-deal CXL region. Rescan
> the dax-bus to retry cxl_dax_region_probe(). No need to bother 'struct
> cxl_region' with a 'dax' flag, it just registers per normal and lets the
> dax-subsystem handle accepting / rejecting.
>
> Now, we do need a mechanism from dax-to-cxl to trigger region removal in
> the DAX_CXL_MODE_REGISTER case (proceed with the hmem registration), but
> that is separate from blocking the attachment of dax to CXL regions.
> Keep all that complexity local to dax.

Okay. To make sure I'm aligned with your suggestion.

It should be something like below in cxl_dax_region_probe():

switch (dax_cxl_mode) {
case DAX_CXL_MODE_DEFER:
return -EPROBE_DEFER;
case DAX_CXL_MODE_REGISTER:
return -ENODEV;
case DAX_CXL_MODE_DROP:
default:
break;
}

Then in the HMEM path, if the SR span is fully covered I will switch to
DAX_CXL_MODE_DROP and trigger a rescan.

Something like:

if (cxl_regions_fully_map(res->start, res->end)) {
dax_cxl_mode = DAX_CXL_MODE_DROP;
bus_rescan_devices(&cxl_bus_type);
} else {
dax_cxl_mode = DAX_CXL_MODE_REGISTER;
cxl_region_teardown(res->start, res->end);
}

hmem_register_device(host, target_nid, res);

cxl_regions_fully_map() will include changes as suggested in Patch 5.

Thanks
Smita