target/arm: support reading of CNT[VCT|FRQ]_EL0 from user-space

Since kernel commit a86bd139f2 (arm64: arch_timer: Enable CNTVCT_EL0
trap..), released in kernel version v4.12, user-space has been able
to read these system registers. As we can't use QEMUTimer's in
linux-user mode we just directly call cpu_get_clock().
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20180625160009.17437-2-alex.bennee@linaro.org
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/arm/helper.c

@@ -2167,11 +2167,32 @@ static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
 };
 
 #else
-/* In user-mode none of the generic timer registers are accessible,
- * and their implementation depends on QEMU_CLOCK_VIRTUAL and qdev gpio outputs,
- * so instead just don't register any of them.
+
+/* In user-mode most of the generic timer registers are inaccessible
+ * however modern kernels (4.12+) allow access to cntvct_el0
  */
+
+static uint64_t gt_virt_cnt_read(CPUARMState *env, const ARMCPRegInfo *ri)
+{
+    /* Currently we have no support for QEMUTimer in linux-user so we
+     * can't call gt_get_countervalue(env), instead we directly
+     * call the lower level functions.
+     */
+    return cpu_get_clock() / GTIMER_SCALE;
+}
+
 static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
+    { .name = "CNTFRQ_EL0", .state = ARM_CP_STATE_AA64,
+      .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 0,
+      .type = ARM_CP_CONST, .access = PL0_R /* no PL1_RW in linux-user */,
+      .fieldoffset = offsetof(CPUARMState, cp15.c14_cntfrq),
+      .resetvalue = NANOSECONDS_PER_SECOND / GTIMER_SCALE,
+    },
+    { .name = "CNTVCT_EL0", .state = ARM_CP_STATE_AA64,
+      .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 2,
+      .access = PL0_R, .type = ARM_CP_NO_RAW | ARM_CP_IO,
+      .readfn = gt_virt_cnt_read,
+    },
     REGINFO_SENTINEL
 };