netdev - Re: [PATCH v2 10/15] cxl: define a driver interface for DPA allocation

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Message-ID: <bc823ebe-b887-bc54-0651-b28362a49a7c@amd.com>
Date: Mon, 19 Aug 2024 16:52:34 +0100
From: Alejandro Lucero Palau <alucerop@....com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.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, richard.hughes@....com
Subject: Re: [PATCH v2 10/15] cxl: define a driver interface for DPA
 allocation


On 8/4/24 19:07, Jonathan Cameron wrote:
> On Mon, 15 Jul 2024 18:28:30 +0100
> alejandro.lucero-palau@....com wrote:
>
>> From: Alejandro Lucero <alucerop@....com>
>>
>> Region creation involves finding available DPA (device-physical-address)
>> capacity to map into HPA (host-physical-address) space. Given the HPA
>> capacity constraint, define an API, cxl_request_dpa(), that has the
>> flexibility to  map the minimum amount of memory the driver needs to
>> operate vs the total possible that can be mapped given HPA availability.
>>
>> Factor out the core of cxl_dpa_alloc, that does free space scanning,
>> into a cxl_dpa_freespace() helper, and use that to balance the capacity
>> available to map vs the @min and @max arguments to cxl_request_dpa.
>>
>> Based on https://lore.kernel.org/linux-cxl/168592149709.1948938.8663425987110396027.stgit@dwillia2-xfh.jf.intel.com/T/#m4271ee49a91615c8af54e3ab20679f8be3099393
>>
> Use the permalink link under these to get shorter links.
> https://lore.kernel.org/linux-cxl/168592158743.1948938.7622563891193802610.stgit@dwillia2-xfh.jf.intel.com/
> goes to the same patch.


I'll do.


>
>> Signed-off-by: Alejandro Lucero <alucerop@....com>
>> Co-developed-by: Dan Williams <dan.j.williams@...el.com>
>
>> +
>> +int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, unsigned long long size)
>> +{
>> +	struct cxl_port *port = cxled_to_port(cxled);
>> +	struct device *dev = &cxled->cxld.dev;
>> +	resource_size_t start, avail, skip;
>> +	int rc;
>> +
>> +	down_write(&cxl_dpa_rwsem);
> Some cleanup.h magic would help here by allowing early returns.
> Needs the scoped lock though to ensure it's released before the
> devm_add_action_or_reset() as I'd guess we will deadlock otherwise
> if that fails.


Yes, I'll try to use it making cleaner code.


>> +	if (cxled->cxld.region) {
>> +		dev_dbg(dev, "EBUSY, decoder attached to %s\n",
>> +			     dev_name(&cxled->cxld.region->dev));
>> +		rc = -EBUSY;
>>   		goto out;
>>   	}
>>   
>> +	if (cxled->cxld.flags & CXL_DECODER_F_ENABLE) {
>> +		dev_dbg(dev, "EBUSY, decoder enabled\n");
>> +		rc = -EBUSY;
>> +		goto out;
>> +	}
>> +
>> +	avail = cxl_dpa_freespace(cxled, &start, &skip);
>> +
>>   	if (size > avail) {
>>   		dev_dbg(dev, "%pa exceeds available %s capacity: %pa\n", &size,
>> -			cxl_decoder_mode_name(cxled->mode), &avail);
>> +			     cxled->mode == CXL_DECODER_RAM ? "ram" : "pmem",
>> +			     &avail);
>>   		rc = -ENOSPC;
>>   		goto out;
>>   	}
>> @@ -550,6 +570,99 @@ int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, unsigned long long size)
>>   	return devm_add_action_or_reset(&port->dev, cxl_dpa_release, cxled);
>>   }
>>   
>> +static int find_free_decoder(struct device *dev, void *data)
>> +{
>> +	struct cxl_endpoint_decoder *cxled;
>> +	struct cxl_port *port;
>> +
>> +	if (!is_endpoint_decoder(dev))
>> +		return 0;
>> +
>> +	cxled = to_cxl_endpoint_decoder(dev);
>> +	port = cxled_to_port(cxled);
>> +
>> +	if (cxled->cxld.id != port->hdm_end + 1) {
>> +		return 0;
> No brackets


Sure.


>> +	}
>> +	return 1;
>> +}
>> +
>> +/**
>> + * cxl_request_dpa - search and reserve DPA given input constraints
>> + * @endpoint: an endpoint port with available decoders
>> + * @mode: DPA operation mode (ram vs pmem)
>> + * @min: the minimum amount of capacity the call needs
>> + * @max: extra capacity to allocate after min is satisfied
>> + *
>> + * Given that a region needs to allocate from limited HPA capacity it
>> + * may be the case that a device has more mappable DPA capacity than
>> + * available HPA. So, the expectation is that @min is a driver known
>> + * value for how much capacity is needed, and @max is based the limit of
>> + * how much HPA space is available for a new region.
> We are going to need a policy control on the max value.
> Otherwise, if you have two devices that support huge capacity and
> not enough space, who gets it will just be a race.
>
> Not a problem for now though!


I agree. If CXL ends up being what we hope, these races will need to be 
better handled.

Thanks!


>> + *
>> + * Returns a pinned cxl_decoder with at least @min bytes of capacity
>> + * reserved, or an error pointer. The caller is also expected to own the
>> + * lifetime of the memdev registration associated with the endpoint to
>> + * pin the decoder registered as well.
>> + */
>
>