[PATCH 1/3] arm64: cpufeature: Force HWCAP to be based on the sysreg visible to user-space

[James Morse]

arm64 advertises hardware features to user-space via HWCAPs, and by
emulating access to the CPUs id registers. The cpufeature code has a
sanitised system-wide view of an id register, and a sanitised user-space
view of an id register, where some features use their 'safe' value
instead of the hardware value.

It is currently possible for a HWCAP to be advertised where the user-space
view of the id register does not show the feature as supported.
Erratum workaround need to remove both the HWCAP, and the feature from
the user-space view of the id register. This involves duplicating the
code, and spreading it over cpufeature.c and cpu_errata.c.

Make the HWCAP code use the user-space view of id registers. This ensures
the values never diverge, and allows erratum workaround to remove HWCAP
by modifying the user-space view of the id register.

Signed-off-by: James Morse <james.morse at arm.com>
---
 arch/arm64/kernel/cpufeature.c | 41 ++++++++++++++++++++++++++--------
 1 file changed, 32 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index af4de817d712..b6a4e97805a4 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1401,17 +1401,40 @@ feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
 	return val >= entry->min_field_value;
 }
 
+static u64
+read_scoped_sysreg(const struct arm64_cpu_capabilities *entry, int scope)
+{
+	WARN_ON(scope == SCOPE_LOCAL_CPU && preemptible());
+	if (scope == SCOPE_SYSTEM)
+		return read_sanitised_ftr_reg(entry->sys_reg);
+	else
+		return __read_sysreg_by_encoding(entry->sys_reg);
+}
+
+static bool
+has_user_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope)
+{
+	int mask;
+	struct arm64_ftr_reg *regp;
+	u64 val = read_scoped_sysreg(entry, scope);
+
+	regp = get_arm64_ftr_reg(entry->sys_reg);
+	if (!regp)
+		return false;
+
+	mask = cpuid_feature_extract_unsigned_field_width(regp->user_mask,
+							  entry->field_pos,
+							  entry->field_width);
+	if (!mask)
+		return false;
+
+	return feature_matches(val, entry);
+}
+
 static bool
 has_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope)
 {
-	u64 val;
-
-	WARN_ON(scope == SCOPE_LOCAL_CPU && preemptible());
-	if (scope == SCOPE_SYSTEM)
-		val = read_sanitised_ftr_reg(entry->sys_reg);
-	else
-		val = __read_sysreg_by_encoding(entry->sys_reg);
-
+	u64 val = read_scoped_sysreg(entry, scope);
 	return feature_matches(val, entry);
 }
 
@@ -2624,7 +2647,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 };
 
 #define HWCAP_CPUID_MATCH(reg, field, width, s, min_value)			\
-		.matches = has_cpuid_feature,					\
+		.matches = has_user_cpuid_feature,					\
 		.sys_reg = reg,							\
 		.field_pos = field,						\
 		.field_width = width,						\
-- 
2.30.2