arm64: compat: Add CNTVCT trap handler

Since people seem to make a point in breaking the userspace visible
counter, we have no choice but to trap the access. We already do this
for 64bit userspace, but this is lacking for compat. Let's provide
the required handler.
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/esr.h

@@ -315,6 +315,9 @@
 					 ESR_ELx_CP15_64_ISS_CRM_MASK | \
 					 ESR_ELx_CP15_64_ISS_DIR_MASK)
 
+#define ESR_ELx_CP15_64_ISS_SYS_CNTVCT	(ESR_ELx_CP15_64_ISS_SYS_VAL(1, 14) | \
+					 ESR_ELx_CP15_64_ISS_DIR_READ)
+
 #ifndef __ASSEMBLY__
 #include <asm/types.h>
 

arch/arm64/kernel/traps.c

@@ -629,7 +629,23 @@ static struct sys64_hook cp15_32_hooks[] = {
 	{},
 };
 
+static void compat_cntvct_read_handler(unsigned int esr, struct pt_regs *regs)
+{
+	int rt = (esr & ESR_ELx_CP15_64_ISS_RT_MASK) >> ESR_ELx_CP15_64_ISS_RT_SHIFT;
+	int rt2 = (esr & ESR_ELx_CP15_64_ISS_RT2_MASK) >> ESR_ELx_CP15_64_ISS_RT2_SHIFT;
+	u64 val = arch_counter_get_cntvct();
+
+	pt_regs_write_reg(regs, rt, lower_32_bits(val));
+	pt_regs_write_reg(regs, rt2, upper_32_bits(val));
+	arm64_compat_skip_faulting_instruction(regs, 4);
+}
+
 static struct sys64_hook cp15_64_hooks[] = {
+	{
+		.esr_mask = ESR_ELx_CP15_64_ISS_SYS_MASK,
+		.esr_val = ESR_ELx_CP15_64_ISS_SYS_CNTVCT,
+		.handler = compat_cntvct_read_handler,
+	},
 	{},
 };