arm64: errata: remove BF16 HWCAP due to incorrect result on Cortex-A510

Cortex-A510's erratum #2658417 causes two BF16 instructions to return the
wrong result in rare circumstances when a pair of A510 CPUs are using
shared neon hardware.

The two instructions affected are BFMMLA and VMMLA, support for these is
indicated by the BF16 HWCAP. Remove it on affected platforms.
Signed-off-by: James Morse <james.morse@arm.com>
Link: https://lore.kernel.org/r/20220909165938.3931307-4-james.morse@arm.com
[catalin.marinas@arm.com: add revision to the Kconfig help; remove .type]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/silicon-errata.rst

@@ -110,6 +110,8 @@ stable kernels.
 +----------------+-----------------+-----------------+-----------------------------+
 | ARM            | Cortex-A510     | #2441009        | ARM64_ERRATUM_2441009       |
 +----------------+-----------------+-----------------+-----------------------------+
+| ARM            | Cortex-A510     | #2658417        | ARM64_ERRATUM_2658417       |
++----------------+-----------------+-----------------+-----------------------------+
 | ARM            | Cortex-A710     | #2119858        | ARM64_ERRATUM_2119858       |
 +----------------+-----------------+-----------------+-----------------------------+
 | ARM            | Cortex-A710     | #2054223        | ARM64_ERRATUM_2054223       |

arch/arm64/Kconfig

@@ -733,6 +733,19 @@ config ARM64_ERRATUM_2077057
 
 	  If unsure, say Y.
 
+config ARM64_ERRATUM_2658417
+	bool "Cortex-A510: 2658417: remove BF16 support due to incorrect result"
+	default y
+	help
+	  This option adds the workaround for ARM Cortex-A510 erratum 2658417.
+	  Affected Cortex-A510 (r0p0 to r1p1) may produce the wrong result for
+	  BFMMLA or VMMLA instructions in rare circumstances when a pair of
+	  A510 CPUs are using shared neon hardware. As the sharing is not
+	  discoverable by the kernel, hide the BF16 HWCAP to indicate that
+	  user-space should not be using these instructions.
+
+	  If unsure, say Y.
+
 config ARM64_ERRATUM_2119858
 	bool "Cortex-A710/X2: 2119858: workaround TRBE overwriting trace data in FILL mode"
 	default y

arch/arm64/kernel/cpu_errata.c

@@ -121,6 +121,22 @@ cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities *__unused)
 	sysreg_clear_set(sctlr_el1, SCTLR_EL1_UCI, 0);
 }
 
+static DEFINE_RAW_SPINLOCK(reg_user_mask_modification);
+static void __maybe_unused
+cpu_clear_bf16_from_user_emulation(const struct arm64_cpu_capabilities *__unused)
+{
+	struct arm64_ftr_reg *regp;
+
+	regp = get_arm64_ftr_reg(SYS_ID_AA64ISAR1_EL1);
+	if (!regp)
+		return;
+
+	raw_spin_lock(&reg_user_mask_modification);
+	if (regp->user_mask & ID_AA64ISAR1_EL1_BF16_MASK)
+		regp->user_mask &= ~ID_AA64ISAR1_EL1_BF16_MASK;
+	raw_spin_unlock(&reg_user_mask_modification);
+}
+
 #define CAP_MIDR_RANGE(model, v_min, r_min, v_max, r_max)	\
 	.matches = is_affected_midr_range,			\
 	.midr_range = MIDR_RANGE(model, v_min, r_min, v_max, r_max)
@@ -691,6 +707,16 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 		CAP_MIDR_RANGE_LIST(broken_aarch32_aes),
 		.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
 	},
+#endif
+#ifdef CONFIG_ARM64_ERRATUM_2658417
+	{
+		.desc = "ARM erratum 2658417",
+		.capability = ARM64_WORKAROUND_2658417,
+		/* Cortex-A510 r0p0 - r1p1 */
+		ERRATA_MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1),
+		MIDR_FIXED(MIDR_CPU_VAR_REV(1,1), BIT(25)),
+		.cpu_enable = cpu_clear_bf16_from_user_emulation,
+	},
 #endif
 	{
 	}

arch/arm64/tools/cpucaps

@@ -68,6 +68,7 @@ WORKAROUND_2038923
 WORKAROUND_2064142
 WORKAROUND_2077057
 WORKAROUND_2457168
+WORKAROUND_2658417
 WORKAROUND_TRBE_OVERWRITE_FILL_MODE
 WORKAROUND_TSB_FLUSH_FAILURE
 WORKAROUND_TRBE_WRITE_OUT_OF_RANGE