Re: [PATCH v5 15/27] cxl: define a driver interface for HPA free space enumeration

[Alejandro Lucero Palau <alucerop@amd.com>]

On 11/22/24 20:45, Ben Cheatham wrote:
> On 11/18/24 10:44 AM, alejandro.lucero-palau@amd.com wrote:
>> From: Alejandro Lucero <alucerop@amd.com>
>>
>> CXL region creation involves allocating capacity from device DPA
>> (device-physical-address space) and assigning it to decode a given HPA
>> (host-physical-address space). Before determining how much DPA to
>> allocate the amount of available HPA must be determined. Also, not all
>> HPA is create equal, some specifically targets RAM, some target PMEM,
>> some is prepared for device-memory flows like HDM-D and HDM-DB, and some
>> is host-only (HDM-H).
>>
>> Wrap all of those concerns into an API that retrieves a root decoder
>> (platform CXL window) that fits the specified constraints and the
>> capacity available for a new region.
>>
>> Based on https://lore.kernel.org/linux-cxl/168592159290.1948938.13522227102445462976.stgit@dwillia2-xfh.jf.intel.com/
>>
>> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
>> Co-developed-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>>   drivers/cxl/core/region.c | 141 ++++++++++++++++++++++++++++++++++++++
>>   drivers/cxl/cxl.h         |   3 +
>>   include/cxl/cxl.h         |   8 +++
>>   3 files changed, 152 insertions(+)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 622e3bb2e04b..d107cc1b4350 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -687,6 +687,147 @@ static int free_hpa(struct cxl_region *cxlr)
>>   	return 0;
>>   }
>>   
>> +struct cxlrd_max_context {
>> +	struct device *host_bridge;
>> +	unsigned long flags;
>> +	resource_size_t max_hpa;
>> +	struct cxl_root_decoder *cxlrd;
>> +};
>> +
>> +static int find_max_hpa(struct device *dev, void *data)
>> +{
>> +	struct cxlrd_max_context *ctx = data;
>> +	struct cxl_switch_decoder *cxlsd;
>> +	struct cxl_root_decoder *cxlrd;
>> +	struct resource *res, *prev;
>> +	struct cxl_decoder *cxld;
>> +	resource_size_t max;
>> +
>> +	if (!is_root_decoder(dev))
>> +		return 0;
>> +
>> +	cxlrd = to_cxl_root_decoder(dev);
>> +	cxlsd = &cxlrd->cxlsd;
>> +	cxld = &cxlsd->cxld;
>> +	if ((cxld->flags & ctx->flags) != ctx->flags) {
>> +		dev_dbg(dev, "%s, flags not matching: %08lx vs %08lx\n",
>> +			__func__, cxld->flags, ctx->flags);
>> +		return 0;
>> +	}
>> +
>> +	/* An accelerator can not be part of an interleaved HPA range. */
> Someone else can weigh in on this, but I would also specify that this is a kernel/driver restriction,
> not a spec one.
>

Right. This was suggested for simplifying the code with the 
current/expected devices, and I bet we will not see such a case for a 
good number of years, but I agree to add that comment.


>> +	if (cxld->interleave_ways != 1) {
>> +		dev_dbg(dev, "%s, interleave_ways not matching\n", __func__);
>> +		return 0;
>> +	}
>> +
>> +	guard(rwsem_read)(&cxl_region_rwsem);
>> +	if (ctx->host_bridge != cxlsd->target[0]->dport_dev) {
>> +		dev_dbg(dev, "%s, host bridge does not match\n", __func__);
>> +		return 0;
>> +	}
> Is this check necessary? I would imagine that there can only be a single
> host bridge above our endpoint since there's also no interleaving?


I guess this is a sanity check. The two variables are given by the 
caller, and there exists the possibility of giving the wrong one for any 
of them. So I think it is a good idea to keep it.


>> +
>> +	/*
>> +	 * Walk the root decoder resource range relying on cxl_region_rwsem to
>> +	 * preclude sibling arrival/departure and find the largest free space
>> +	 * gap.
>> +	 */
>> +	lockdep_assert_held_read(&cxl_region_rwsem);
>> +	max = 0;
>> +	res = cxlrd->res->child;
>> +	if (!res)
>> +		max = resource_size(cxlrd->res);
>> +	else
>> +		max = 0;
>> +
>> +	for (prev = NULL; res; prev = res, res = res->sibling) {
>> +		struct resource *next = res->sibling;
>> +		resource_size_t free = 0;
>> +
>> +		if (!prev && res->start > cxlrd->res->start) {
>> +			free = res->start - cxlrd->res->start;
>> +			max = max(free, max);
>> +		}
>> +		if (prev && res->start > prev->end + 1) {
>> +			free = res->start - prev->end + 1;
>> +			max = max(free, max);
>> +		}
>> +		if (next && res->end + 1 < next->start) {
>> +			free = next->start - res->end + 1;
>> +			max = max(free, max);
>> +		}
>> +		if (!next && res->end + 1 < cxlrd->res->end + 1) {
>> +			free = cxlrd->res->end + 1 - res->end + 1;
>> +			max = max(free, max);
>> +		}
>> +	}
>> +
>> +	dev_dbg(CXLRD_DEV(cxlrd), "%s, found %pa bytes of free space\n",
>> +		__func__, &max);
>> +	if (max > ctx->max_hpa) {
>> +		if (ctx->cxlrd)
>> +			put_device(CXLRD_DEV(ctx->cxlrd));
>> +		get_device(CXLRD_DEV(cxlrd));
>> +		ctx->cxlrd = cxlrd;
>> +		ctx->max_hpa = max;
>> +		dev_dbg(CXLRD_DEV(cxlrd), "%s, found %pa bytes of free space\n",
>> +			__func__, &max);
>> +	}
>> +	return 0;
>> +}
>> +
>> +/**
>> + * cxl_get_hpa_freespace - find a root decoder with free capacity per constraints
>> + * @endpoint: an endpoint that is mapped by the returned decoder
>> + * @flags: CXL_DECODER_F flags for selecting RAM vs PMEM, and HDM-H vs HDM-D[B]
>> + * @max_avail_contig: output parameter of max contiguous bytes available in the
>> + *		      returned decoder
>> + *
>> + * The return tuple of a 'struct cxl_root_decoder' and 'bytes available (@max)'
> The (@max) part should be (@max_avail_contig), no?


Right. I'll fix it.


>> + * is a point in time snapshot. If by the time the caller goes to use this root
>> + * decoder's capacity the capacity is reduced then caller needs to loop and
>> + * retry.
>> + *
>> + * The returned root decoder has an elevated reference count that needs to be
>> + * put with put_device(cxlrd_dev(cxlrd)). Locking context is with
>> + * cxl_{acquire,release}_endpoint(), that ensures removal of the root decoder
>> + * does not race.
>> + */
>> +struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
>> +					       unsigned long flags,
>> +					       resource_size_t *max_avail_contig)
>> +{
>> +	struct cxl_port *endpoint = cxlmd->endpoint;
>> +	struct cxlrd_max_context ctx = {
>> +		.host_bridge = endpoint->host_bridge,
>> +		.flags = flags,
>> +	};
>> +	struct cxl_port *root_port;
>> +	struct cxl_root *root __free(put_cxl_root) = find_cxl_root(endpoint);
>> +
>> +	if (!is_cxl_endpoint(endpoint)) {
>> +		dev_dbg(&endpoint->dev, "hpa requestor is not an endpoint\n");
>> +		return ERR_PTR(-EINVAL);
>> +	}
>> +
>> +	if (!root) {
>> +		dev_dbg(&endpoint->dev, "endpoint can not be related to a root port\n");
> This message makes it seem like there's a problem with the endpoint, not the hierarchy (at least to me).
> Maybe something like "can't find root port associated with endpoint" or "can't find root port above endpoint" instead?


Well, there is a problem for sure, but I can not see why your suggestion 
improves things here. It is not easy to really know what is the problem 
without a real case triggering it, so IMO blaming one or another is a 
blind choice by now.


>> +		return ERR_PTR(-ENXIO);
>> +	}
>> +
>> +	root_port = &root->port;
>> +	down_read(&cxl_region_rwsem);
>> +	device_for_each_child(&root_port->dev, &ctx, find_max_hpa);
>> +	up_read(&cxl_region_rwsem);
>> +
>> +	if (!ctx.cxlrd)
>> +		return ERR_PTR(-ENOMEM);
>> +
>> +	*max_avail_contig = ctx.max_hpa;
>> +	return ctx.cxlrd;
>> +}
>> +EXPORT_SYMBOL_NS_GPL(cxl_get_hpa_freespace, CXL);
>> +
>>   static ssize_t size_store(struct device *dev, struct device_attribute *attr,
>>   			  const char *buf, size_t len)
>>   {
>> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
>> index e5f918be6fe4..1e0e797b9303 100644
>> --- a/drivers/cxl/cxl.h
>> +++ b/drivers/cxl/cxl.h
>> @@ -776,6 +776,9 @@ static inline void cxl_dport_init_ras_reporting(struct cxl_dport *dport,
>>   struct cxl_decoder *to_cxl_decoder(struct device *dev);
>>   struct cxl_root_decoder *to_cxl_root_decoder(struct device *dev);
>>   struct cxl_switch_decoder *to_cxl_switch_decoder(struct device *dev);
>> +
>> +#define CXLRD_DEV(cxlrd) (&(cxlrd)->cxlsd.cxld.dev)
>> +
>>   struct cxl_endpoint_decoder *to_cxl_endpoint_decoder(struct device *dev);
>>   bool is_root_decoder(struct device *dev);
>>   bool is_switch_decoder(struct device *dev);
>> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
>> index 5608ed0f5f15..4508b5c186e8 100644
>> --- a/include/cxl/cxl.h
>> +++ b/include/cxl/cxl.h
>> @@ -7,6 +7,10 @@
>>   #include <linux/ioport.h>
>>   #include <linux/pci.h>
>>   
>> +#define CXL_DECODER_F_RAM   BIT(0)
>> +#define CXL_DECODER_F_PMEM  BIT(1)
>> +#define CXL_DECODER_F_TYPE2 BIT(2)
>> +
>>   enum cxl_resource {
>>   	CXL_RES_DPA,
>>   	CXL_RES_RAM,
>> @@ -59,4 +63,8 @@ int cxl_release_resource(struct cxl_dev_state *cxlds, enum cxl_resource type);
>>   void cxl_set_media_ready(struct cxl_dev_state *cxlds);
>>   struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
>>   				       struct cxl_dev_state *cxlds);
>> +struct cxl_port;
>> +struct cxl_root_decoder *cxl_get_hpa_freespace(struct cxl_memdev *cxlmd,
>> +					       unsigned long flags,
>> +					       resource_size_t *max);
>>   #endif