[Qemu-devel] [RFC PATCH] target/arm: support reading of CNTVCT_EL0 from user-space

["Alex Bennée" <alex.bennee@linaro.org>]

Since kernel commit a86bd139f2 (arm64: arch_timer: Enable CNTVCT_EL0
trap..) user-space has been able to read this system register. This
patch enables access to that register although currently it always
returns 0 as we don't yet have a mechanism for managing timers in
linux-user mode.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 target/arm/helper.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index b14fdab140..8244badd63 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2121,11 +2121,25 @@ 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).
+     */
+    return 0;
+}
+
 static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
+    { .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
 };
 
-- 
2.17.0