arm64: Rework checks for broken Cavium HW in the PI code

Calling into the MIDR checking framework from the PI code has recently
become much harder, due to the new fancy "multi-MIDR" support that
relies on tables being populated at boot time, but not that early that
they are available to the PI code. There are additional issues with
this framework, as the code really isn't position independend *at all*.

This leads to some ugly breakages, as reported by Ada.

It so appears that the only reason for the PI code to call into the
MIDR checking code is to cope with The Most Broken ARM64 System Ever,
aka Cavium ThunderX, which cannot deal with nG attributes that result
of the combination of KASLR and KPTI as a consequence of Erratum 27456.

Duplicate the check for the erratum in the PI code, removing the
dependency on the bulk of the MIDR checking framework. This allows
dropping that same check from kaslr_requires_kpti(), as the KPTI code
already relies on the ARM64_WORKAROUND_CAVIUM_27456 cap.

Fixes: c8c2647 ("arm64: Make  _midr_in_range_list() an exported function")
Reported-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/3d97e45a-23cf-419b-9b6f-140b4d88de7b@arm.com
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Cc: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250418093129.1755739-1-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/include/asm/mmu.h

@@ -94,17 +94,6 @@ static inline bool kaslr_requires_kpti(void)
 			return false;
 	}
 
-	/*
-	 * Systems affected by Cavium erratum 24756 are incompatible
-	 * with KPTI.
-	 */
-	if (IS_ENABLED(CONFIG_CAVIUM_ERRATUM_27456)) {
-		extern const struct midr_range cavium_erratum_27456_cpus[];
-
-		if (is_midr_in_range_list(cavium_erratum_27456_cpus))
-			return false;
-	}
-
 	return true;
 }
 

arch/arm64/kernel/cpu_errata.c

@@ -335,7 +335,7 @@ static const struct midr_range cavium_erratum_23154_cpus[] = {
 #endif
 
 #ifdef CONFIG_CAVIUM_ERRATUM_27456
-const struct midr_range cavium_erratum_27456_cpus[] = {
+static const struct midr_range cavium_erratum_27456_cpus[] = {
 	/* Cavium ThunderX, T88 pass 1.x - 2.1 */
 	MIDR_RANGE(MIDR_THUNDERX, 0, 0, 1, 1),
 	/* Cavium ThunderX, T81 pass 1.0 */

arch/arm64/kernel/image-vars.h

@@ -47,10 +47,6 @@ PROVIDE(__pi_id_aa64smfr0_override	= id_aa64smfr0_override);
 PROVIDE(__pi_id_aa64zfr0_override	= id_aa64zfr0_override);
 PROVIDE(__pi_arm64_sw_feature_override	= arm64_sw_feature_override);
 PROVIDE(__pi_arm64_use_ng_mappings	= arm64_use_ng_mappings);
-#ifdef CONFIG_CAVIUM_ERRATUM_27456
-PROVIDE(__pi_cavium_erratum_27456_cpus	= cavium_erratum_27456_cpus);
-PROVIDE(__pi_is_midr_in_range_list	= is_midr_in_range_list);
-#endif
 PROVIDE(__pi__ctype			= _ctype);
 PROVIDE(__pi_memstart_offset_seed	= memstart_offset_seed);
 

arch/arm64/kernel/pi/map_kernel.c

@@ -207,6 +207,29 @@ static void __init map_fdt(u64 fdt)
 	dsb(ishst);
 }
 
+/*
+ * PI version of the Cavium Eratum 27456 detection, which makes it
+ * impossible to use non-global mappings.
+ */
+static bool __init ng_mappings_allowed(void)
+{
+	static const struct midr_range cavium_erratum_27456_cpus[] __initconst = {
+		/* Cavium ThunderX, T88 pass 1.x - 2.1 */
+		MIDR_RANGE(MIDR_THUNDERX, 0, 0, 1, 1),
+		/* Cavium ThunderX, T81 pass 1.0 */
+		MIDR_REV(MIDR_THUNDERX_81XX, 0, 0),
+		{},
+	};
+
+	for (const struct midr_range *r = cavium_erratum_27456_cpus; r->model; r++) {
+		if (midr_is_cpu_model_range(read_cpuid_id(), r->model,
+					    r->rv_min, r->rv_max))
+			return false;
+	}
+
+	return true;
+}
+
 asmlinkage void __init early_map_kernel(u64 boot_status, void *fdt)
 {
 	static char const chosen_str[] __initconst = "/chosen";
@@ -246,7 +269,7 @@ asmlinkage void __init early_map_kernel(u64 boot_status, void *fdt)
 		u64 kaslr_seed = kaslr_early_init(fdt, chosen);
 
 		if (kaslr_seed && kaslr_requires_kpti())
-			arm64_use_ng_mappings = true;
+			arm64_use_ng_mappings = ng_mappings_allowed();
 
 		kaslr_offset |= kaslr_seed & ~(MIN_KIMG_ALIGN - 1);
 	}