[Qemu-devel] [PATCH] target-arm: Make the counter tick relative to cntfrq

[Andrew Jeffery]

The use of GTIMER_SCALE assumes the clock feeding the generic timer is
62.5MHz for all platforms. This is untrue in general, for example the
ASPEED AST2600 feeds the counter with either an 800 or 1200MHz clock,
and CNTFRQ is configured appropriately by u-boot.

To cope with these values we need to take care of the quantization
caused by the clock scaling in terms of nanoseconds per clock by
calculating an effective frequency such that NANOSECONDS_PER_SECOND is
an integer multiple of the effective frequency. Failing to account for
the quantisation leads to sticky behaviour in the VM as the guest timer
subsystems account for the difference between delay time and the counter
value.

Signed-off-by: Andrew Jeffery <address@hidden>
---
The timer rate assumptions seemed unusual, so I'm not sure if this patch is way
off-base or not. However it does make the AST2600 u-boot and kernel behave
correctly.

 target/arm/helper.c | 51 +++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 47 insertions(+), 4 deletions(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index b74c23a9bc08..35d14c183630 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2277,6 +2277,34 @@ static const ARMCPRegInfo v6k_cp_reginfo[] = {
 
 #ifndef CONFIG_USER_ONLY
 
+static void gt_recalc_timer(ARMCPU *cpu, int timeridx);
+static void gt_cntfrq_write(CPUARMState *env, const ARMCPRegInfo *ri,
+                            uint64_t value)
+{
+    uint64_t scale;
+    ARMCPU *cpu;
+    int i;
+
+    raw_write(env, ri, value);
+
+    /* Fix up the timer scaling */
+    cpu = env_archcpu(env);
+    scale = MAX(1, NANOSECONDS_PER_SECOND / value);
+    for (i = 0; i < NUM_GTIMERS; i++) {
+        if (!cpu->gt_timer[i]) {
+            continue;
+        }
+
+        cpu->gt_timer[i]->scale = scale;
+        gt_recalc_timer(cpu, i);
+    }
+}
+
+static void gt_cntfrq_reset(CPUARMState *env, const ARMCPRegInfo *opaque)
+{
+    gt_cntfrq_write(env, opaque, opaque->resetvalue);
+}
+
 static CPAccessResult gt_cntfrq_access(CPUARMState *env, const ARMCPRegInfo 
*ri,
                                        bool isread)
 {
@@ -2405,9 +2433,21 @@ static CPAccessResult gt_stimer_access(CPUARMState *env,
     }
 }
 
+static uint64_t gt_calc_tick(CPUARMState *env, uint64_t ns)
+{
+    /*
+     * Deal with quantized clock scaling by calculating the effective frequency
+     * in terms of the timer scale.
+     */
+    uint32_t scale = MAX(1, NANOSECONDS_PER_SECOND / env->cp15.c14_cntfrq);
+    uint64_t effective = NANOSECONDS_PER_SECOND / scale;
+
+    return muldiv64(ns, effective, NANOSECONDS_PER_SECOND);
+}
+
 static uint64_t gt_get_countervalue(CPUARMState *env)
 {
-    return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) / GTIMER_SCALE;
+    return gt_calc_tick(env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL));
 }
 
 static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
@@ -2443,8 +2483,8 @@ static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
          * set the timer for as far in the future as possible. When the
          * timer expires we will reset the timer for any remaining period.
          */
-        if (nexttick > INT64_MAX / GTIMER_SCALE) {
-            nexttick = INT64_MAX / GTIMER_SCALE;
+        if (nexttick > INT64_MAX / cpu->gt_timer[timeridx]->scale) {
+            nexttick = INT64_MAX / cpu->gt_timer[timeridx]->scale;
         }
         timer_mod(cpu->gt_timer[timeridx], nexttick);
         trace_arm_gt_recalc(timeridx, irqstate, nexttick);
@@ -2686,13 +2726,16 @@ static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
     { .name = "CNTFRQ", .cp = 15, .crn = 14, .crm = 0, .opc1 = 0, .opc2 = 0,
       .type = ARM_CP_ALIAS,
       .access = PL1_RW | PL0_R, .accessfn = gt_cntfrq_access,
+      .writefn = gt_cntfrq_write,
       .fieldoffset = offsetoflow32(CPUARMState, cp15.c14_cntfrq),
     },
     { .name = "CNTFRQ_EL0", .state = ARM_CP_STATE_AA64,
       .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 0,
       .access = PL1_RW | PL0_R, .accessfn = gt_cntfrq_access,
+      .writefn = gt_cntfrq_write,
       .fieldoffset = offsetof(CPUARMState, cp15.c14_cntfrq),
       .resetvalue = (1000 * 1000 * 1000) / GTIMER_SCALE,
+      .resetfn = gt_cntfrq_reset,
     },
     /* overall control: mostly access permissions */
     { .name = "CNTKCTL", .state = ARM_CP_STATE_BOTH,
@@ -2875,7 +2918,7 @@ static uint64_t gt_virt_cnt_read(CPUARMState *env, const 
ARMCPRegInfo *ri)
      * can't call gt_get_countervalue(env), instead we directly
      * call the lower level functions.
      */
-    return cpu_get_clock() / GTIMER_SCALE;
+    return gt_calc_tick(env, cpu_get_clock());
 }
 
 static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
-- 
2.20.1