[Problem ?] cxl list show nothing after reboot Re: [PATCH v2] cxl/pci: Fix disabling memory if DVSEC CXL Range does not match a CFMWS window

[Zhijian Li (Fujitsu)]

Robert, Dan

It's noticed that 'cxl list' show nothing after a reboot in v6.8.(A fresh boot works)
The git bisection pointed to this commit.

Haven't investigated it deeply, I'm wondering if it's a QEMU problem or
something wrong with this patch.


Reproduce step:

1. Start a cxl QEMU VM
2. cxl list works
cxl list
[
   {
     "memdev":"mem0",
     "ram_size":2147483648,
     "serial":0,
     "host":"0000:54:00.0"
   },
   {
     "memdev":"mem1",
     "pmem_size":2147483648,
     "serial":0,
     "host":"0000:36:00.0"
   }
]

3. reboot VM
4. cxl list show nothing and has following dmesg

cxl list
[
]
   Warning: no matching devices found

...

[    6.249188]  pci0000:53: host supports CXL
[    6.258168]  pci0000:35: host supports CXL
[    6.490568] cxl_pci 0000:54:00.0: Range register decodes outside platform defined CXL ranges.
[    6.494298] cxl_mem mem0: endpoint3 failed probe
[    6.506072] cxl_pci 0000:36:00.0: Range register decodes outside platform defined CXL ranges.
[    6.515092] cxl_mem mem1: endpoint3 failed probe
[   12.181188] kauditd_printk_skb: 18 callbacks suppressed


Thanks
Zhijian


On 17/02/2024 00:01, Robert Richter wrote:
> The Linux CXL subsystem is built on the assumption that HPA == SPA.
> That is, the host physical address (HPA) the HDM decoder registers are
> programmed with are system physical addresses (SPA).
> 
> During HDM decoder setup, the DVSEC CXL range registers (cxl-3.1,
> 8.1.3.8) are checked if the memory is enabled and the CXL range is in
> a HPA window that is described in a CFMWS structure of the CXL host
> bridge (cxl-3.1, 9.18.1.3).
> 
> Now, if the HPA is not an SPA, the CXL range does not match a CFMWS
> window and the CXL memory range will be disabled then. The HDM decoder
> stops working which causes system memory being disabled and further a
> system hang during HDM decoder initialization, typically when a CXL
> enabled kernel boots.
> 
> Prevent a system hang and do not disable the HDM decoder if the
> decoder's CXL range is not found in a CFMWS window.
> 
> Note the change only fixes a hardware hang, but does not implement
> HPA/SPA translation. Support for this can be added in a follow on
> patch series.
> 
> Signed-off-by: Robert Richter <rrichter@xxxxxxx>
> ---
>   drivers/cxl/core/pci.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index a0e7ed5ae25f..18616ca873e5 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -478,8 +478,8 @@ int cxl_hdm_decode_init(struct cxl_dev_state *cxlds, struct cxl_hdm *cxlhdm,
>   	}
>   
>   	if (!allowed) {
> -		cxl_set_mem_enable(cxlds, 0);
> -		info->mem_enabled = 0;
> +		dev_err(dev, "Range register decodes outside platform defined CXL ranges.\n");
> +		return -ENXIO;
>   	}
>   
>   	/*