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De Francesco" , linux-cxl@vger.kernel.org Cc: Davidlohr Bueso , Jonathan Cameron , Alison Schofield , Vishal Verma , Ira Weiny , Dan Williams , Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Gregory Price , Robert Richter , Cheatham Benjamin References: <20251006155836.791418-1-fabio.m.de.francesco@linux.intel.com> <20251006155836.791418-3-fabio.m.de.francesco@linux.intel.com> Content-Language: en-US From: Dave Jiang In-Reply-To: <20251006155836.791418-3-fabio.m.de.francesco@linux.intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/6/25 8:58 AM, Fabio M. De Francesco wrote: > On a x86 platform with a low memory hole (LHM), the BIOS may publish > CFMWS that describes a system physical address (SPA) range that > typically is only a subset of the corresponding CXL intermediate switch > and endpoint decoder's host physical address (HPA) ranges. The CFMWS > range never intersects the LHM and so the driver instantiates a root > decoder whose HPA range size doesn't fully contain the matching switch > and endpoint decoders' HPA ranges.[1] > > To construct regions and attach decoders, the driver needs to match root > decoders and regions with endpoint decoders. The process fails and > returns errors because the driver is not designed to deal with SPA > ranges which are smaller than the corresponding hardware decoders HPA > ranges. > > Introduce two functions that indirectly detect the presence of x86 LMH > and allow the matching between a root decoder or an already constructed > region with a corresponding intermediate switch or endpoint decoder to > enable the construction of a region and the subsequent attachment of the > same decoders to that region. > > These functions return true when SPA/HPA misalignments due to LMH's are > detected under specific conditions: > > - Both the SPA and HPA ranges must start at LMH_CFMWS_RANGE_START (i.e., > 0x0 on x86 with LMH's). > - The SPA range's size is less than HPA's. > - The SPA range's size is less than 4G. > - The HPA range's size is aligned to the NIW * 256M rule. > > Also introduce a function that adjusts the range end of a region to be > constructed and the DPA range's end of the endpoint decoders that will > be later attached to that region. > > [1] commit 7a81173f3 ("cxl: Documentation/driver-api/cxl: Describe the x86 Low Memory Hole solution") I don't think this is the right hash. c5dca38633da ("cxl: Documentation/driver-api/cxl: Describe the x86 Low Memory Hole solution") > > Cc: Alison Schofield > Cc: Dan Williams > Cc: Dave Jiang > Cc: Ira Weiny > Signed-off-by: Fabio M. De Francesco > --- > drivers/cxl/Kconfig | 4 ++ > drivers/cxl/core/Makefile | 1 + > drivers/cxl/core/platform_quirks.c | 99 ++++++++++++++++++++++++++++++ > drivers/cxl/core/platform_quirks.h | 33 ++++++++++ > 4 files changed, 137 insertions(+) > create mode 100644 drivers/cxl/core/platform_quirks.c > create mode 100644 drivers/cxl/core/platform_quirks.h > > diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig > index 48b7314afdb8..03c0583bc9a3 100644 > --- a/drivers/cxl/Kconfig > +++ b/drivers/cxl/Kconfig > @@ -211,6 +211,10 @@ config CXL_REGION > > If unsure say 'y' > > +config CXL_PLATFORM_QUIRKS > + def_bool y > + depends on CXL_REGION > + > config CXL_REGION_INVALIDATION_TEST > bool "CXL: Region Cache Management Bypass (TEST)" > depends on CXL_REGION > diff --git a/drivers/cxl/core/Makefile b/drivers/cxl/core/Makefile > index 5ad8fef210b5..1684e46b8709 100644 > --- a/drivers/cxl/core/Makefile > +++ b/drivers/cxl/core/Makefile > @@ -17,6 +17,7 @@ cxl_core-y += cdat.o > cxl_core-y += ras.o > cxl_core-$(CONFIG_TRACING) += trace.o > cxl_core-$(CONFIG_CXL_REGION) += region.o > +cxl_core-$(CONFIG_CXL_PLATFORM_QUIRKS) += platform_quirks.o > cxl_core-$(CONFIG_CXL_MCE) += mce.o > cxl_core-$(CONFIG_CXL_FEATURES) += features.o > cxl_core-$(CONFIG_CXL_EDAC_MEM_FEATURES) += edac.o > diff --git a/drivers/cxl/core/platform_quirks.c b/drivers/cxl/core/platform_quirks.c > new file mode 100644 > index 000000000000..7e76e392b1ae > --- /dev/null > +++ b/drivers/cxl/core/platform_quirks.c > @@ -0,0 +1,99 @@ > +// SPDX-License-Identifier: GPL-2.0-only > + > +#include > +#include "platform_quirks.h" > +#include "cxlmem.h" > +#include "core.h" > + > +/* Start of CFMWS range that end before x86 Low Memory Holes */ > +#define LMH_CFMWS_RANGE_START 0x0ULL > + > +/** > + * platform_cxlrd_matches_cxled() - Platform quirk to match CXL Root and > + * Endpoint Decoders. It allows matching on platforms with LMH's. > + * @cxlrd: The Root Decoder against which @cxled is tested for matching. > + * @cxled: The Endpoint Decoder to be tested for matching @cxlrd. > + * > + * platform_cxlrd_matches_cxled() is typically called from the > + * match_*_by_range() functions in region.c. It checks if an endpoint decoder > + * matches a given root decoder and returns true to allow the driver to succeed > + * in the construction of regions where it would otherwise fail for the presence > + * of a Low Memory Hole (see Documentation/driver-api/cxl/conventions.rst). > + * > + * In x86 platforms with LMH's, the CFMWS ranges never intersect the LMH, the > + * endpoint decoder's HPA range size is always guaranteed aligned to NIW*256MB > + * and also typically larger than the matching root decoder's, and the root > + * decoder's range end is at an address that is necessarily less than SZ_4G > + * (i.e., the Hole is in Low Memory - this function doesn't deal with other > + * kinds of holes). > + * > + * Return: true if an endpoint matches a root decoder, else false. > + */ > +bool platform_cxlrd_matches_cxled(const struct cxl_root_decoder *cxlrd, > + const struct cxl_endpoint_decoder *cxled) > +{ > + const struct range *rd_r, *sd_r; > + int align; > + > + rd_r = &cxlrd->cxlsd.cxld.hpa_range; > + sd_r = &cxled->cxld.hpa_range; > + align = cxled->cxld.interleave_ways * SZ_256M; > + > + if (rd_r->start == LMH_CFMWS_RANGE_START && > + rd_r->start == sd_r->start && rd_r->end < sd_r->end && Break this into 2 lines to make it more readable > + rd_r->end < (LMH_CFMWS_RANGE_START + SZ_4G) && > + IS_ALIGNED(range_len(sd_r), align)) > + return true; > + > + return false; > +} > + > +/** > + * platform_region_matches_cxld() - Platform quirk to match a CXL Region and a > + * Switch or Endpoint Decoder. It allows matching on platforms with LMH's. > + * @p: Region Params against which @cxled is matched. > + * @cxld: Switch or Endpoint Decoder to be tested for matching @p. > + * > + * Similar to platform_cxlrd_matches_cxled(), it matches regions and > + * decoders on platforms with LMH's. > + * > + * Return: true if a Decoder matches a Region, else false. > + */ > +bool platform_region_matches_cxld(const struct cxl_region_params *p, > + const struct cxl_decoder *cxld) > +{ > + const struct range *r = &cxld->hpa_range; > + const struct resource *res = p->res; > + int align = cxld->interleave_ways * SZ_256M; > + > + if (res->start == LMH_CFMWS_RANGE_START && res->start == r->start && > + res->end < r->end && res->end < (LMH_CFMWS_RANGE_START + SZ_4G) && Break the first and this line into 2 lines each with a single compare to make it more readable > + IS_ALIGNED(range_len(r), align)) > + return true; > + > + return false; > +} > + > +void platform_res_adjust(struct resource *res, > + struct cxl_endpoint_decoder *cxled, > + const struct cxl_root_decoder *cxlrd) > +{ > + if (!platform_cxlrd_matches_cxled(cxlrd, cxled)) > + return; > + > + guard(rwsem_write)(&cxl_rwsem.dpa); > + dev_dbg(cxled_to_memdev(cxled)->dev.parent, > + "Low Memory Hole detected. Resources were (%s: %pr, %pr)\n", > + dev_name(&cxled->cxld.dev), res, cxled->dpa_res); > + if (res) { > + /* Trim region resource overlap with LMH */ > + res->end = cxlrd->res->end; > + } > + /* Match endpoint decoder's DPA resource to root decoder's */ > + cxled->dpa_res->end = > + cxled->dpa_res->start + this can be on the same line as the first DJ > + resource_size(cxlrd->res) / cxled->cxld.interleave_ways - 1; > + dev_info(cxled_to_memdev(cxled)->dev.parent, > + "Resources have been adjusted for LMH (%s: %pr, %pr)\n", > + dev_name(&cxled->cxld.dev), res, cxled->dpa_res); > +} > diff --git a/drivers/cxl/core/platform_quirks.h b/drivers/cxl/core/platform_quirks.h > new file mode 100644 > index 000000000000..a15592b4e90e > --- /dev/null > +++ b/drivers/cxl/core/platform_quirks.h > @@ -0,0 +1,33 @@ > +/* SPDX-License-Identifier: GPL-2.0-only */ > + > +#include "cxl.h" > + > +#ifdef CONFIG_CXL_PLATFORM_QUIRKS > +bool platform_cxlrd_matches_cxled(const struct cxl_root_decoder *cxlrd, > + const struct cxl_endpoint_decoder *cxled); > +bool platform_region_matches_cxld(const struct cxl_region_params *p, > + const struct cxl_decoder *cxld); > +void platform_res_adjust(struct resource *res, > + struct cxl_endpoint_decoder *cxled, > + const struct cxl_root_decoder *cxlrd); > +#else > +static inline bool > +platform_root_decoder_contains(const struct cxl_root_decoder *cxlrd, > + const struct cxl_endpoint_decoder *cxled) > +{ > + return false; > +} > + > +static inline bool > +platform_region_matches_cxld(const struct cxl_region_params *p, > + const struct cxl_decoder *cxld) > +{ > + return false; > +} > + > +inline void platform_res_adjust(struct resource *res, > + struct cxl_endpoint_decoder *cxled, > + const struct cxl_root_decoder *cxlrd) > +{ > +} > +#endif /* CONFIG_CXL_PLATFORM_QUIRKS */