netdev - Re: [PATCH v5 24/27] cxl: add region flag for precluding a device memory to be used for dax

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Message-ID: <229773bc-b81d-7e72-0b4b-3aeb6a7306d8@amd.com>
Date: Wed, 27 Nov 2024 16:59:16 +0000
From: Alejandro Lucero Palau <alucerop@....com>
To: Ben Cheatham <benjamin.cheatham@....com>, alejandro.lucero-palau@....com
Cc: linux-cxl@...r.kernel.org, netdev@...r.kernel.org,
 dan.j.williams@...el.com, martin.habets@...inx.com, edward.cree@....com,
 davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com
Subject: Re: [PATCH v5 24/27] cxl: add region flag for precluding a device
 memory to be used for dax


On 11/22/24 20:46, Ben Cheatham wrote:
> On 11/18/24 10:44 AM, alejandro.lucero-palau@....com wrote:
>> From: Alejandro Lucero <alucerop@....com>
>>
>> By definition a type2 cxl device will use the host managed memory for
>> specific functionality, therefore it should not be available to other
>> uses. However, a dax interface could be just good enough in some cases.
>>
>> Add a flag to a cxl region for specifically state to not create a dax
>> device. Allow a Type2 driver to set that flag at region creation time.
>>
>> Signed-off-by: Alejandro Lucero <alucerop@....com>
>> ---
>>   drivers/cxl/core/region.c | 10 +++++++++-
>>   drivers/cxl/cxl.h         |  3 +++
>>   drivers/cxl/cxlmem.h      |  3 ++-
>>   include/cxl/cxl.h         |  3 ++-
>>   4 files changed, 16 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 70549d42c2e3..eff3ad788077 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -3558,7 +3558,8 @@ __construct_new_region(struct cxl_root_decoder *cxlrd,
>>    * cxl_region driver.
>>    */
>>   struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
>> -				     struct cxl_endpoint_decoder *cxled)
>> +				     struct cxl_endpoint_decoder *cxled,
>> +				     bool avoid_dax)
>>   {
>>   	struct cxl_region *cxlr;
>>   
>> @@ -3574,6 +3575,10 @@ struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
>>   		drop_region(cxlr);
>>   		return ERR_PTR(-ENODEV);
>>   	}
>> +
>> +	if (avoid_dax)
>> +		set_bit(CXL_REGION_F_AVOID_DAX, &cxlr->flags);
>> +
>>   	return cxlr;
>>   }
>>   EXPORT_SYMBOL_NS_GPL(cxl_create_region, CXL);
>> @@ -3713,6 +3718,9 @@ static int cxl_region_probe(struct device *dev)
>>   	case CXL_DECODER_PMEM:
>>   		return devm_cxl_add_pmem_region(cxlr);
>>   	case CXL_DECODER_RAM:
>> +		if (test_bit(CXL_REGION_F_AVOID_DAX, &cxlr->flags))
>> +			return 0;
> I think it's possible for a type 2 device to have pmem as well, and
> it looks like these are the only two options at the moment, so I would
> just move this check to before the switch statement.


Yes, that was also suggested in previous patchsets and I forgot.

I'll do it for v6.


>> +
>>   		/*
>>   		 * The region can not be manged by CXL if any portion of
>>   		 * it is already online as 'System RAM'
>> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
>> index 1e0e797b9303..ee3385db5663 100644
>> --- a/drivers/cxl/cxl.h
>> +++ b/drivers/cxl/cxl.h
>> @@ -512,6 +512,9 @@ struct cxl_region_params {
>>    */
>>   #define CXL_REGION_F_NEEDS_RESET 1
>>   
>> +/* Allow Type2 drivers to specify if a dax region should not be created. */
>> +#define CXL_REGION_F_AVOID_DAX 2
>> +
> I would like to see flags such that the device could choose the region type
> (system ram, device-dax, or none). I think that adding the ability
> for device-dax would add a patch or two, so that may be a good follow up
> patch.


Not sure the system ram option makes sense when this code can be executed.

Anyway, as you say, let's leave this for a follow up.

Thanks


>>   /**
>>    * struct cxl_region - CXL region
>>    * @dev: This region's device
>> diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
>> index 9d874f1cb3bf..cc2e2a295f3d 100644
>> --- a/drivers/cxl/cxlmem.h
>> +++ b/drivers/cxl/cxlmem.h
>> @@ -875,5 +875,6 @@ struct seq_file;
>>   struct dentry *cxl_debugfs_create_dir(const char *dir);
>>   void cxl_dpa_debug(struct seq_file *file, struct cxl_dev_state *cxlds);
>>   struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
>> -				     struct cxl_endpoint_decoder *cxled);
>> +				     struct cxl_endpoint_decoder *cxled,
>> +				     bool avoid_dax);
>>   #endif /* __CXL_MEM_H__ */
>> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
>> index d295af4f5f9e..2a8ebabfc1dd 100644
>> --- a/include/cxl/cxl.h
>> +++ b/include/cxl/cxl.h
>> @@ -73,7 +73,8 @@ struct cxl_endpoint_decoder *cxl_request_dpa(struct cxl_memdev *cxlmd,
>>   					     resource_size_t max);
>>   int cxl_dpa_free(struct cxl_endpoint_decoder *cxled);
>>   struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
>> -				     struct cxl_endpoint_decoder *cxled);
>> +				     struct cxl_endpoint_decoder *cxled,
>> +				     bool avoid_dax);
>>   
>>   int cxl_accel_region_detach(struct cxl_endpoint_decoder *cxled);
>>   #endif