arm64: Fixup user features at boot time

For ARM64_WORKAROUND_2658417, we use a cpu_enable() callback to hide the
ID_AA64ISAR1_EL1.BF16 ID register field. This is a little awkward as
CPUs may attempt to apply the workaround concurrently, requiring that we
protect the bulk of the callback with a raw_spinlock, and requiring some
pointless work every time a CPU is subsequently hotplugged in.

This patch makes this a little simpler by handling the masking once at
boot time. A new user_feature_fixup() function is called at the start of
setup_user_features() to mask the feature, matching the style of
elf_hwcap_fixup(). The ARM64_WORKAROUND_2658417 cpucap is added to
cpucap_is_possible() so that code can be elided entirely when this is
not possible.

Note that the ARM64_WORKAROUND_2658417 capability is matched with
ERRATA_MIDR_RANGE(), which implicitly gives the capability a
ARM64_CPUCAP_LOCAL_CPU_ERRATUM type, which forbids the late onlining of
a CPU with the erratum if the erratum was not present at boot time.
Therefore this patch doesn't change the behaviour for late onlining.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/cpucaps.h

@@ -40,6 +40,8 @@ cpucap_is_possible(const unsigned int cap)
 		return IS_ENABLED(CONFIG_ARM64_BTI);
 	case ARM64_HAS_TLB_RANGE:
 		return IS_ENABLED(CONFIG_ARM64_TLB_RANGE);
+	case ARM64_WORKAROUND_2658417:
+		return IS_ENABLED(CONFIG_ARM64_ERRATUM_2658417);
 	}
 
 	return true;

arch/arm64/kernel/cpu_errata.c

@@ -121,22 +121,6 @@ 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)
@@ -727,7 +711,6 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 		/* 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
 #ifdef CONFIG_AMPERE_ERRATUM_AC03_CPU_38

arch/arm64/kernel/cpufeature.c

@@ -2190,6 +2190,17 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
 }
 #endif /* CONFIG_ARM64_MTE */
 
+static void user_feature_fixup(void)
+{
+	if (cpus_have_cap(ARM64_WORKAROUND_2658417)) {
+		struct arm64_ftr_reg *regp;
+
+		regp = get_arm64_ftr_reg(SYS_ID_AA64ISAR1_EL1);
+		if (regp)
+			regp->user_mask &= ~ID_AA64ISAR1_EL1_BF16_MASK;
+	}
+}
+
 static void elf_hwcap_fixup(void)
 {
 #ifdef CONFIG_ARM64_ERRATUM_1742098
@@ -3357,6 +3368,8 @@ void __init setup_system_features(void)
 
 void __init setup_user_features(void)
 {
+	user_feature_fixup();
+
 	setup_elf_hwcaps(arm64_elf_hwcaps);
 
 	if (system_supports_32bit_el0()) {