cxl/region: Manage CPU caches relative to DPA invalidation events

A "DPA invalidation event" is any scenario where the contents of a DPA
(Device Physical Address) is modified in a way that is incoherent with
CPU caches, or if the HPA (Host Physical Address) to DPA association
changes due to a remapping event.

PMEM security events like Unlock and Passphrase Secure Erase already
manage caches through LIBNVDIMM, so that leaves HPA to DPA remap events
that need cache management by the CXL core. Those only happen when the
boot time CXL configuration has changed. That event occurs when
userspace attaches an endpoint decoder to a region configuration, and
that region is subsequently activated.

The implications of not invalidating caches between remap events is that
reads from the region at different points in time may return different
results due to stale cached data from the previous HPA to DPA mapping.
Without a guarantee that the region contents after cxl_region_probe()
are written before being read (a layering-violation assumption that
cxl_region_probe() can not make) the CXL subsystem needs to ensure that
reads that precede writes see consistent results.

A CONFIG_CXL_REGION_INVALIDATION_TEST option is added to support debug
and unit testing of the CXL implementation in QEMU or other environments
where cpu_cache_has_invalidate_memregion() returns false. This may prove
too restrictive for QEMU where the HDM decoders are emulated, but in
that case the CXL subsystem needs some new mechanism / indication that
the HDM decoder is emulated and not a passthrough of real hardware.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/166993222098.1995348.16604163596374520890.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/Kconfig

@@ -111,4 +111,22 @@ config CXL_REGION
 	select MEMREGION
 	select GET_FREE_REGION
 
+config CXL_REGION_INVALIDATION_TEST
+	bool "CXL: Region Cache Management Bypass (TEST)"
+	depends on CXL_REGION
+	help
+	  CXL Region management and security operations potentially invalidate
+	  the content of CPU caches without notifiying those caches to
+	  invalidate the affected cachelines. The CXL Region driver attempts
+	  to invalidate caches when those events occur.  If that invalidation
+	  fails the region will fail to enable.  Reasons for cache
+	  invalidation failure are due to the CPU not providing a cache
+	  invalidation mechanism. For example usage of wbinvd is restricted to
+	  bare metal x86. However, for testing purposes toggling this option
+	  can disable that data integrity safety and proceed with enabling
+	  regions when there might be conflicting contents in the CPU cache.
+
+	  If unsure, or if this kernel is meant for production environments,
+	  say N.
+
 endif

drivers/cxl/core/region.c

@@ -1403,6 +1403,8 @@ static int attach_target(struct cxl_region *cxlr, const char *decoder, int pos)
 		goto out;
 	down_read(&cxl_dpa_rwsem);
 	rc = cxl_region_attach(cxlr, to_cxl_endpoint_decoder(dev), pos);
+	if (rc == 0)
+		set_bit(CXL_REGION_F_INCOHERENT, &cxlr->flags);
 	up_read(&cxl_dpa_rwsem);
 	up_write(&cxl_region_rwsem);
 out:
@@ -1900,6 +1902,30 @@ err:
 	return rc;
 }
 
+static int cxl_region_invalidate_memregion(struct cxl_region *cxlr)
+{
+	if (!test_bit(CXL_REGION_F_INCOHERENT, &cxlr->flags))
+		return 0;
+
+	if (!cpu_cache_has_invalidate_memregion()) {
+		if (IS_ENABLED(CONFIG_CXL_REGION_INVALIDATION_TEST)) {
+			dev_warn(
+				&cxlr->dev,
+				"Bypassing cpu_cache_invalidate_memergion() for testing!\n");
+			clear_bit(CXL_REGION_F_INCOHERENT, &cxlr->flags);
+			return 0;
+		} else {
+			dev_err(&cxlr->dev,
+				"Failed to synchronize CPU cache state\n");
+			return -ENXIO;
+		}
+	}
+
+	cpu_cache_invalidate_memregion(IORES_DESC_CXL);
+	clear_bit(CXL_REGION_F_INCOHERENT, &cxlr->flags);
+	return 0;
+}
+
 static int cxl_region_probe(struct device *dev)
 {
 	struct cxl_region *cxlr = to_cxl_region(dev);
@@ -1915,12 +1941,16 @@ static int cxl_region_probe(struct device *dev)
 	if (p->state < CXL_CONFIG_COMMIT) {
 		dev_dbg(&cxlr->dev, "config state: %d\n", p->state);
 		rc = -ENXIO;
+		goto out;
 	}
 
+	rc = cxl_region_invalidate_memregion(cxlr);
+
 	/*
 	 * From this point on any path that changes the region's state away from
 	 * CXL_CONFIG_COMMIT is also responsible for releasing the driver.
 	 */
+out:
 	up_read(&cxl_region_rwsem);
 
 	if (rc)
@@ -1953,4 +1983,5 @@ void cxl_region_exit(void)
 }
 
 MODULE_IMPORT_NS(CXL);
+MODULE_IMPORT_NS(DEVMEM);
 MODULE_ALIAS_CXL(CXL_DEVICE_REGION);

drivers/cxl/cxl.h

@@ -380,12 +380,19 @@ struct cxl_region_params {
 	int nr_targets;
 };
 
+/*
+ * Flag whether this region needs to have its HPA span synchronized with
+ * CPU cache state at region activation time.
+ */
+#define CXL_REGION_F_INCOHERENT 0
+
 /**
  * struct cxl_region - CXL region
  * @dev: This region's device
  * @id: This region's id. Id is globally unique across all regions
  * @mode: Endpoint decoder allocation / access mode
  * @type: Endpoint decoder target type
+ * @flags: Region state flags
  * @params: active + config params for the region
  */
 struct cxl_region {
@@ -393,6 +400,7 @@ struct cxl_region {
 	int id;
 	enum cxl_decoder_mode mode;
 	enum cxl_decoder_type type;
+	unsigned long flags;
 	struct cxl_region_params params;
 };
 

drivers/cxl/security.c

@@ -120,17 +120,12 @@ static int cxl_pmem_security_unlock(struct nvdimm *nvdimm,
 	u8 pass[NVDIMM_PASSPHRASE_LEN];
 	int rc;
 
-	if (!cpu_cache_has_invalidate_memregion())
-		return -EINVAL;
-
 	memcpy(pass, key_data->data, NVDIMM_PASSPHRASE_LEN);
 	rc = cxl_mbox_send_cmd(cxlds, CXL_MBOX_OP_UNLOCK,
 			       pass, NVDIMM_PASSPHRASE_LEN, NULL, 0);
 	if (rc < 0)
 		return rc;
 
-	/* DIMM unlocked, invalidate all CPU caches before we read it */
-	cpu_cache_invalidate_memregion(IORES_DESC_PERSISTENT_MEMORY);
 	return 0;
 }
 
@@ -144,21 +139,14 @@ static int cxl_pmem_security_passphrase_erase(struct nvdimm *nvdimm,
 	struct cxl_pass_erase erase;
 	int rc;
 
-	if (!cpu_cache_has_invalidate_memregion())
-		return -EINVAL;
-
 	erase.type = ptype == NVDIMM_MASTER ?
 		CXL_PMEM_SEC_PASS_MASTER : CXL_PMEM_SEC_PASS_USER;
 	memcpy(erase.pass, key->data, NVDIMM_PASSPHRASE_LEN);
-	/* Flush all cache before we erase mem device */
-	cpu_cache_invalidate_memregion(IORES_DESC_PERSISTENT_MEMORY);
 	rc = cxl_mbox_send_cmd(cxlds, CXL_MBOX_OP_PASSPHRASE_SECURE_ERASE,
 			       &erase, sizeof(erase), NULL, 0);
 	if (rc < 0)
 		return rc;
 
-	/* mem device erased, invalidate all CPU caches before data is read */
-	cpu_cache_invalidate_memregion(IORES_DESC_PERSISTENT_MEMORY);
 	return 0;
 }
 
@@ -173,5 +161,3 @@ static const struct nvdimm_security_ops __cxl_security_ops = {
 };
 
 const struct nvdimm_security_ops *cxl_security_ops = &__cxl_security_ops;
-
-MODULE_IMPORT_NS(DEVMEM);