arm64: cpufeature: add cpus_have_final_cap()

When cpus_have_const_cap() was originally introduced it was intended to
be safe in hyp context, where it is not safe to access the cpu_hwcaps
array as cpus_have_cap() did. For more details see commit:

  a4023f68 ("arm64: Add hypervisor safe helper for checking constant capabilities")

We then made use of cpus_have_const_cap() throughout the kernel.

Subsequently, we had to defer updating the static_key associated with
each capability in order to avoid lockdep complaints. To avoid breaking
kernel-wide usage of cpus_have_const_cap(), this was updated to fall
back to the cpu_hwcaps array if called before the static_keys were
updated. As the kvm hyp code was only called later than this, the
fallback is redundant but not functionally harmful. For more details,
see commit:

  63a1e1c9 ("arm64/cpufeature: don't use mutex in bringup path")

Today we have more users of cpus_have_const_cap() which are only called
once the relevant static keys are initialized, and it would be
beneficial to avoid the redundant code.

To that end, this patch adds a new cpus_have_final_cap(), helper which
is intend to be used in code which is only run once capabilities have
been finalized, and will never check the cpus_hwcap array. This helps
the compiler to generate better code as it no longer needs to generate
code to address and test the cpus_hwcap array. To help catch misuse,
cpus_have_final_cap() will BUG() if called before capabilities are
finalized.

In hyp context, BUG() will result in a hyp panic, but the specific BUG()
instance will not be identified in the usual way.

Comments are added to the various cpus_have_*_cap() helpers to describe
the constraints on when they can be used. For clarity cpus_have_cap() is
moved above the other helpers. Similarly the helpers are updated to use
system_capabilities_finalized() consistently, and this is made
__always_inline as required by its new callers.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: 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/cpufeature.h

@@ -390,14 +390,16 @@ unsigned long cpu_get_elf_hwcap2(void);
 #define cpu_set_named_feature(name) cpu_set_feature(cpu_feature(name))
 #define cpu_have_named_feature(name) cpu_have_feature(cpu_feature(name))
 
-/* System capability check for constant caps */
-static __always_inline bool __cpus_have_const_cap(int num)
+static __always_inline bool system_capabilities_finalized(void)
 {
-	if (num >= ARM64_NCAPS)
-		return false;
-	return static_branch_unlikely(&cpu_hwcap_keys[num]);
+	return static_branch_likely(&arm64_const_caps_ready);
 }
 
+/*
+ * Test for a capability with a runtime check.
+ *
+ * Before the capability is detected, this returns false.
+ */
 static inline bool cpus_have_cap(unsigned int num)
 {
 	if (num >= ARM64_NCAPS)
@@ -405,14 +407,53 @@ static inline bool cpus_have_cap(unsigned int num)
 	return test_bit(num, cpu_hwcaps);
 }
 
+/*
+ * Test for a capability without a runtime check.
+ *
+ * Before capabilities are finalized, this returns false.
+ * After capabilities are finalized, this is patched to avoid a runtime check.
+ *
+ * @num must be a compile-time constant.
+ */
+static __always_inline bool __cpus_have_const_cap(int num)
+{
+	if (num >= ARM64_NCAPS)
+		return false;
+	return static_branch_unlikely(&cpu_hwcap_keys[num]);
+}
+
+/*
+ * Test for a capability, possibly with a runtime check.
+ *
+ * Before capabilities are finalized, this behaves as cpus_have_cap().
+ * After capabilities are finalized, this is patched to avoid a runtime check.
+ *
+ * @num must be a compile-time constant.
+ */
 static __always_inline bool cpus_have_const_cap(int num)
 {
-	if (static_branch_likely(&arm64_const_caps_ready))
+	if (system_capabilities_finalized())
 		return __cpus_have_const_cap(num);
 	else
 		return cpus_have_cap(num);
 }
 
+/*
+ * Test for a capability without a runtime check.
+ *
+ * Before capabilities are finalized, this will BUG().
+ * After capabilities are finalized, this is patched to avoid a runtime check.
+ *
+ * @num must be a compile-time constant.
+ */
+static __always_inline bool cpus_have_final_cap(int num)
+{
+	if (system_capabilities_finalized())
+		return __cpus_have_const_cap(num);
+	else
+		BUG();
+}
+
 static inline void cpus_set_cap(unsigned int num)
 {
 	if (num >= ARM64_NCAPS) {
@@ -613,11 +654,6 @@ static inline bool system_has_prio_mask_debugging(void)
 	       system_uses_irq_prio_masking();
 }
 
-static inline bool system_capabilities_finalized(void)
-{
-	return static_branch_likely(&arm64_const_caps_ready);
-}
-
 #define ARM64_BP_HARDEN_UNKNOWN		-1
 #define ARM64_BP_HARDEN_WA_NEEDED	0
 #define ARM64_BP_HARDEN_NOT_REQUIRED	1