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[3/4] target/arm: Prepare generic timer for per-platform CNTFRQ

Message ID 20191128054527.25450-4-andrew@aj.id.au (mailing list archive)
State New, archived
Headers show
Series Expose GT CNTFRQ as a CPU property to support AST2600 | expand

Commit Message

Andrew Jeffery Nov. 28, 2019, 5:45 a.m. UTC
The ASPEED AST2600 clocks the generic timer at the rate of HPLL. On
recent firmwares this is at 1125MHz, which is considerably quicker than
the assumed 62.5MHz of the current generic timer implementation. The
delta between the value as read from CNTFRQ and the true rate of the
underlying QEMUTimer leads to sticky behaviour in AST2600 guests.

Add a feature-gated property exposing CNTFRQ for ARM CPUs providing the
generic timer. This allows platforms to configure CNTFRQ (and the
associated QEMUTimer) to the appropriate frequency prior to starting the
guest.

As the platform can now determine the rate of CNTFRQ we're exposed to
limitations of QEMUTimer that didn't previously materialise: In the
course of emulation we need to arbitrarily and accurately convert
between guest ticks and time, but we're constrained by QEMUTimer's use
of an integer scaling factor. Its effect is QEMUTimer cannot exactly
capture the period of frequencies that do not cleanly divide
NANOSECONDS_PER_SECOND for scaling ticks to time. As such, provide an
equally inaccurate scaling factor for scaling time to ticks so at least
an self-consistent inverse relationship holds.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
---
 target/arm/cpu.c    | 43 +++++++++++++++++++++++++++++++++----------
 target/arm/cpu.h    | 18 ++++++++++++++++++
 target/arm/helper.c |  9 ++++++++-
 3 files changed, 59 insertions(+), 11 deletions(-)
diff mbox series

Patch

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 5698a74061bb..f186019a77fd 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -974,10 +974,12 @@  static void arm_cpu_initfn(Object *obj)
     if (tcg_enabled()) {
         cpu->psci_version = 2; /* TCG implements PSCI 0.2 */
     }
-
-    cpu->gt_cntfrq = NANOSECONDS_PER_SECOND / GTIMER_SCALE;
 }
 
+static Property arm_cpu_gt_cntfrq_property =
+            DEFINE_PROP_UINT64("cntfrq", ARMCPU, gt_cntfrq,
+                               NANOSECONDS_PER_SECOND / GTIMER_SCALE);
+
 static Property arm_cpu_reset_cbar_property =
             DEFINE_PROP_UINT64("reset-cbar", ARMCPU, reset_cbar, 0);
 
@@ -1174,6 +1176,11 @@  void arm_cpu_post_init(Object *obj)
 
     qdev_property_add_static(DEVICE(obj), &arm_cpu_cfgend_property,
                              &error_abort);
+
+    if (arm_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER)) {
+        qdev_property_add_static(DEVICE(cpu), &arm_cpu_gt_cntfrq_property,
+                                 &error_abort);
+    }
 }
 
 static void arm_cpu_finalizefn(Object *obj)
@@ -1253,14 +1260,30 @@  static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
         }
     }
 
-    cpu->gt_timer[GTIMER_PHYS] = timer_new(QEMU_CLOCK_VIRTUAL, GTIMER_SCALE,
-                                           arm_gt_ptimer_cb, cpu);
-    cpu->gt_timer[GTIMER_VIRT] = timer_new(QEMU_CLOCK_VIRTUAL, GTIMER_SCALE,
-                                           arm_gt_vtimer_cb, cpu);
-    cpu->gt_timer[GTIMER_HYP] = timer_new(QEMU_CLOCK_VIRTUAL, GTIMER_SCALE,
-                                          arm_gt_htimer_cb, cpu);
-    cpu->gt_timer[GTIMER_SEC] = timer_new(QEMU_CLOCK_VIRTUAL, GTIMER_SCALE,
-                                          arm_gt_stimer_cb, cpu);
+
+    {
+        uint64_t scale;
+
+        if (arm_feature(env, ARM_FEATURE_GENERIC_TIMER)) {
+            if (!cpu->gt_cntfrq) {
+                error_setg(errp, "Invalid CNTFRQ: %"PRId64"Hz",
+                           cpu->gt_cntfrq);
+                return;
+            }
+            scale = gt_cntfrq_period_ns(cpu);
+        } else {
+            scale = GTIMER_SCALE;
+        }
+
+        cpu->gt_timer[GTIMER_PHYS] = timer_new(QEMU_CLOCK_VIRTUAL, scale,
+                                               arm_gt_ptimer_cb, cpu);
+        cpu->gt_timer[GTIMER_VIRT] = timer_new(QEMU_CLOCK_VIRTUAL, scale,
+                                               arm_gt_vtimer_cb, cpu);
+        cpu->gt_timer[GTIMER_HYP] = timer_new(QEMU_CLOCK_VIRTUAL, scale,
+                                              arm_gt_htimer_cb, cpu);
+        cpu->gt_timer[GTIMER_SEC] = timer_new(QEMU_CLOCK_VIRTUAL, scale,
+                                              arm_gt_stimer_cb, cpu);
+    }
 #endif
 
     cpu_exec_realizefn(cs, &local_err);
diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 666c03871fdf..0bcd13dcac81 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -939,6 +939,24 @@  struct ARMCPU {
 
 static inline unsigned int gt_cntfrq_period_ns(ARMCPU *cpu)
 {
+    /*
+     * The exact approach to calculating guest ticks is:
+     *
+     *     muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), cpu->gt_cntfrq,
+     *              NANOSECONDS_PER_SECOND);
+     *
+     * We don't do that. Rather we intentionally use integer division
+     * truncation below and in the caller for the conversion of host monotonic
+     * time to guest ticks to provide the exact inverse for the semantics of
+     * the QEMUTimer scale factor. QEMUTimer's scale facter is an integer, so
+     * it loses precision when representing frequencies where
+     * `(NANOSECONDS_PER_SECOND % cpu->gt_cntfrq) > 0` holds. Failing to
+     * provide an exact inverse leads to scheduling timers with negative
+     * periods, which in turn leads to sticky behaviour in the guest.
+     *
+     * Finally, CNTFRQ is effectively capped at 1GHz to ensure our scale factor
+     * cannot become zero.
+     */
     /* XXX: Could include qemu/timer.h to get NANOSECONDS_PER_SECOND? */
     const unsigned int ns_per_s = 1000 * 1000 * 1000;
     return ns_per_s > cpu->gt_cntfrq ? ns_per_s / cpu->gt_cntfrq : 1;
diff --git a/target/arm/helper.c b/target/arm/helper.c
index 1cc0551081a0..79e278e78291 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2683,6 +2683,13 @@  void arm_gt_stimer_cb(void *opaque)
     gt_recalc_timer(cpu, GTIMER_SEC);
 }
 
+static void arm_gt_cntfrq_reset(CPUARMState *env, const ARMCPRegInfo *opaque)
+{
+    ARMCPU *cpu = env_archcpu(env);
+
+    cpu->env.cp15.c14_cntfrq = cpu->gt_cntfrq;
+}
+
 static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
     /* Note that CNTFRQ is purely reads-as-written for the benefit
      * of software; writing it doesn't actually change the timer frequency.
@@ -2697,7 +2704,7 @@  static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
       .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 0,
       .access = PL1_RW | PL0_R, .accessfn = gt_cntfrq_access,
       .fieldoffset = offsetof(CPUARMState, cp15.c14_cntfrq),
-      .resetvalue = (1000 * 1000 * 1000) / GTIMER_SCALE,
+      .resetfn = arm_gt_cntfrq_reset,
     },
     /* overall control: mostly access permissions */
     { .name = "CNTKCTL", .state = ARM_CP_STATE_BOTH,