arm64: proton-pack: Expose whether the branchy loop k value

Add a helper to expose the k value of the branchy loop. This is needed
by the BPF JIT to generate the mitigation sequence in BPF programs.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Author: James Morse

arch/arm64/include/asm/spectre.h

@@ -97,6 +97,7 @@ enum mitigation_state arm64_get_meltdown_state(void);
 
 enum mitigation_state arm64_get_spectre_bhb_state(void);
 bool is_spectre_bhb_affected(const struct arm64_cpu_capabilities *entry, int scope);
+u8 get_spectre_bhb_loop_value(void);
 bool is_spectre_bhb_fw_mitigated(void);
 void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *__unused);
 bool try_emulate_el1_ssbs(struct pt_regs *regs, u32 instr);

arch/arm64/kernel/proton-pack.c

@@ -999,6 +999,11 @@ bool is_spectre_bhb_affected(const struct arm64_cpu_capabilities *entry,
 	return true;
 }
 
+u8 get_spectre_bhb_loop_value(void)
+{
+	return max_bhb_k;
+}
+
 static void this_cpu_set_vectors(enum arm64_bp_harden_el1_vectors slot)
 {
 	const char *v = arm64_get_bp_hardening_vector(slot);