Re: [PATCH v9 00/19] DCD: Add support for Dynamic Capacity Devices (DCD)

[Ira Weiny <ira.weiny@intel.com>]

Fan Ni wrote:
> On Sun, Apr 13, 2025 at 05:52:08PM -0500, Ira Weiny wrote:
> > A git tree of this series can be found here:
> > 
> > 	https://github.com/weiny2/linux-kernel/tree/dcd-v6-2025-04-13
> > 
> > This is now based on 6.15-rc2.
> > 
> > Due to the stagnation of solid requirements for users of DCD I do not
> > plan to rev this work in Q2 of 2025 and possibly beyond.
> > 
> > It is anticipated that this will support at least the initial
> > implementation of DCD devices, if and when they appear in the ecosystem.
> > The patch set should be reviewed with the limited set of functionality in
> > mind.  Additional functionality can be added as devices support them.
> > 
> > It is strongly encouraged for individuals or companies wishing to bring
> > DCD devices to market review this set with the customer use cases they
> > have in mind.
> 
> Hi Ira,
> thanks for sending it out.
> 
> I have not got a chance to check the code or test it extensively.
> 
> I tried to test one specific case and hit issue.
> 
> I tried to add some DC extents to the extent list on the device when the
> VM is launched by hacking qemu like below,
> 
> diff --git a/hw/mem/cxl_type3.c b/hw/mem/cxl_type3.c
> index 87fa308495..4049fc8dd9 100644
> --- a/hw/mem/cxl_type3.c
> +++ b/hw/mem/cxl_type3.c
> @@ -826,6 +826,11 @@ static bool cxl_create_dc_regions(CXLType3Dev *ct3d, Error **errp)
>      QTAILQ_INIT(&ct3d->dc.extents);
>      QTAILQ_INIT(&ct3d->dc.extents_pending);
>  
> +    cxl_insert_extent_to_extent_list(&ct3d->dc.extents, 0,
> +                                     CXL_CAPACITY_MULTIPLIER, NULL, 0);
> +    ct3d->dc.total_extent_count = 1;
> +    ct3_set_region_block_backed(ct3d, 0, CXL_CAPACITY_MULTIPLIER);
> +
>      return true;
>  }
> 
> 
> Then after the VM is launched, I tried to create a DC region with
> commmand: cxl create-region -m mem0 -d decoder0.0 -s 1G -t
> dynamic_ram_a.
> 
> It works fine. As you can see below, the region is created and the
> extent is showing correctly.
> 
> root@debian:~# cxl list -r region0 -N
> [
>   {
>     "region":"region0",
>     "resource":79725330432,
>     "size":1073741824,
>     "interleave_ways":1,
>     "interleave_granularity":256,
>     "decode_state":"commit",
>     "extents":[
>       {
>         "offset":0,
>         "length":268435456,
>         "uuid":"00000000-0000-0000-0000-000000000000"
>       }
>     ]
>   }
> ]
> 
> 
> However, after that, I tried to create a dax device as below, it failed.
> 
> root@debian:~# daxctl create-device -r region0 -v
> libdaxctl: __dax_regions_init: no dax regions found via: /sys/class/dax
> error creating devices: No such device or address
> created 0 devices
> root@debian:~# 
> 
> root@debian:~# ls /sys/class/dax 
> ls: cannot access '/sys/class/dax': No such file or directory

Have you update daxctl with cxl-cli?

I was confused by this lack of /sys/class/dax and checked with Vishal.  He
says this is legacy.

I have /sys/bus/dax and that works fine for me with the latest daxctl
built from the ndctl code I sent out:

https://github.com/weiny2/ndctl/tree/dcd-region3-2025-04-13

Could you build and use the executables from that version?

Ira

> 
> The dmesg shows the really_probe function returns early as resource
> presents before probe as below,
> 
> [ 1745.505068] cxl_core:devm_cxl_add_dax_region:3251: cxl_region region0: region0: register dax_region0
> [ 1745.506063] cxl_pci:__cxl_pci_mbox_send_cmd:263: cxl_pci 0000:0d:00.0: Sending command: 0x4801
> [ 1745.506953] cxl_pci:cxl_pci_mbox_wait_for_doorbell:74: cxl_pci 0000:0d:00.0: Doorbell wait took 0ms
> [ 1745.507911] cxl_core:__cxl_process_extent_list:1802: cxl_pci 0000:0d:00.0: Got extent list 0-0 of 1 generation Num:0
> [ 1745.508958] cxl_core:__cxl_process_extent_list:1815: cxl_pci 0000:0d:00.0: Processing extent 0/1
> [ 1745.509843] cxl_core:cxl_validate_extent:975: cxl_pci 0000:0d:00.0: DC extent DPA [range 0x0000000000000000-0x000000000fffffff] (DCR:[range 0x0000000000000000-0x000000007fffffff])(00000000-0000-0000-0000-000000000000)
> [ 1745.511748] cxl_core:__cxl_dpa_to_region:2869: cxl decoder2.0: dpa:0x0 mapped in region:region0
> [ 1745.512626] cxl_core:cxl_add_extent:460: cxl decoder2.0: Checking ED ([mem 0x00000000-0x3fffffff flags 0x80000200]) for extent [range 0x0000000000000000-0x000000000fffffff]
> [ 1745.514143] cxl_core:cxl_add_extent:492: cxl decoder2.0: Add extent [range 0x0000000000000000-0x000000000fffffff] (00000000-0000-0000-0000-000000000000)
> [ 1745.515485] cxl_core:online_region_extent:176:  extent0.0: region extent HPA [range 0x0000000000000000-0x000000000fffffff]
> [ 1745.516576] cxl_core:cxlr_notify_extent:285: cxl dax_region0: Trying notify: type 0 HPA [range 0x0000000000000000-0x000000000fffffff]
> [ 1745.517768] cxl_core:cxl_bus_probe:2087: cxl_region region0: probe: 0
> [ 1745.524984] cxl dax_region0: Resources present before probing
> 
> 
> btw, I hit the same issue with the previous verson also.
> 
> Fan

[snip]