Re: [PATCH v7 15/28] cxl: define a driver interface for HPA free space enumeration

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

On 12/12/24 18:09, Simon Horman wrote:
> On Mon, Dec 09, 2024 at 06:54:16PM +0000, 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 | 154 ++++++++++++++++++++++++++++++++++++++
>>   drivers/cxl/cxl.h         |   3 +
>>   include/cxl/cxl.h         |   8 ++
>>   3 files changed, 165 insertions(+)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 967132b49832..77af6a59f4b5 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -687,6 +687,160 @@ 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;
>> +	}
>> +
>> +	/*
>> +	 * The CXL specs do not forbid an accelerator being part of an
>> +	 * interleaved HPA range, but it is unlikely and because it helps
>> +	 * simplifying the code, we assume this being the case by now.
>> +	 */
>> +	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;
>> +	}
>> +
>> +	/*
>> +	 * 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;
>> +
>> +		/*
>> +		 * Sanity check for preventing arithmetic problems below as a
>> +		 * resource with size 0 could imply using the end field below
>> +		 * when set to unsigned zero - 1 or all f in hex.
>> +		 */
>> +		if (!resource_size(prev))
>> +			continue;
> Hi Alejandro and Dan,
>
> Below it is assumed that prev may be null.
> But above resource_size will dereference it unconditionally.
> That doesn't seem right.


Hi Simon,


I added this code in the last version for addressing concerns regarding 
the resource being with 0 size, so it is me the one to put the blame on!


I'll fix that for v8.


Thanks!


> Flagged by Smatch.
>
>> +
>> +		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;
>> +}
> ...