arm/hvf: Add a WFI handler

Sleep on WFI until the VTIMER is due but allow ourselves to be woken
up on IPI.

In this implementation IPI is blocked on the CPU thread at startup and
pselect() is used to atomically unblock the signal and begin sleeping.
The signal is sent unconditionally so there's no need to worry about
races between actually sleeping and the "we think we're sleeping"
state. It may lead to an extra wakeup but that's better than missing
it entirely.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Signed-off-by: Alexander Graf <agraf@csgraf.de>
Acked-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20210916155404.86958-6-agraf@csgraf.de
[agraf: Remove unused 'set' variable, always advance PC on WFX trap,
        support vm stop / continue operations and cntv offsets]
Signed-off-by: Alexander Graf <agraf@csgraf.de>
Acked-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

accel/hvf/hvf-accel-ops.c

@@ -370,15 +370,14 @@ static int hvf_init_vcpu(CPUState *cpu)
     cpu->hvf = g_malloc0(sizeof(*cpu->hvf));
 
     /* init cpu signals */
-    sigset_t set;
     struct sigaction sigact;
 
     memset(&sigact, 0, sizeof(sigact));
     sigact.sa_handler = dummy_signal;
     sigaction(SIG_IPI, &sigact, NULL);
 
-    pthread_sigmask(SIG_BLOCK, NULL, &set);
-    sigdelset(&set, SIG_IPI);
+    pthread_sigmask(SIG_BLOCK, NULL, &cpu->hvf->unblock_ipi_mask);
+    sigdelset(&cpu->hvf->unblock_ipi_mask, SIG_IPI);
 
 #ifdef __aarch64__
     r = hv_vcpu_create(&cpu->hvf->fd, (hv_vcpu_exit_t **)&cpu->hvf->exit, NULL);

include/sysemu/hvf_int.h

@@ -52,6 +52,7 @@ struct hvf_vcpu_state {
     uint64_t fd;
     void *exit;
     bool vtimer_masked;
+    sigset_t unblock_ipi_mask;
 };
 
 void assert_hvf_ok(hv_return_t ret);

target/arm/hvf/hvf.c

@@ -2,6 +2,7 @@
  * QEMU Hypervisor.framework support for Apple Silicon
 
  * Copyright 2020 Alexander Graf <agraf@csgraf.de>
+ * Copyright 2020 Google LLC
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
@@ -490,6 +491,7 @@ int hvf_arch_init_vcpu(CPUState *cpu)
 
 void hvf_kick_vcpu_thread(CPUState *cpu)
 {
+    cpus_kick_thread(cpu);
     hv_vcpus_exit(&cpu->hvf->fd, 1);
 }
 
@@ -608,6 +610,80 @@ static uint64_t hvf_vtimer_val_raw(void)
     return mach_absolute_time() - hvf_state->vtimer_offset;
 }
 
+static uint64_t hvf_vtimer_val(void)
+{
+    if (!runstate_is_running()) {
+        /* VM is paused, the vtimer value is in vtimer.vtimer_val */
+        return vtimer.vtimer_val;
+    }
+
+    return hvf_vtimer_val_raw();
+}
+
+static void hvf_wait_for_ipi(CPUState *cpu, struct timespec *ts)
+{
+    /*
+     * Use pselect to sleep so that other threads can IPI us while we're
+     * sleeping.
+     */
+    qatomic_mb_set(&cpu->thread_kicked, false);
+    qemu_mutex_unlock_iothread();
+    pselect(0, 0, 0, 0, ts, &cpu->hvf->unblock_ipi_mask);
+    qemu_mutex_lock_iothread();
+}
+
+static void hvf_wfi(CPUState *cpu)
+{
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+    struct timespec ts;
+    hv_return_t r;
+    uint64_t ctl;
+    uint64_t cval;
+    int64_t ticks_to_sleep;
+    uint64_t seconds;
+    uint64_t nanos;
+    uint32_t cntfrq;
+
+    if (cpu->interrupt_request & (CPU_INTERRUPT_HARD | CPU_INTERRUPT_FIQ)) {
+        /* Interrupt pending, no need to wait */
+        return;
+    }
+
+    r = hv_vcpu_get_sys_reg(cpu->hvf->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
+    assert_hvf_ok(r);
+
+    if (!(ctl & 1) || (ctl & 2)) {
+        /* Timer disabled or masked, just wait for an IPI. */
+        hvf_wait_for_ipi(cpu, NULL);
+        return;
+    }
+
+    r = hv_vcpu_get_sys_reg(cpu->hvf->fd, HV_SYS_REG_CNTV_CVAL_EL0, &cval);
+    assert_hvf_ok(r);
+
+    ticks_to_sleep = cval - hvf_vtimer_val();
+    if (ticks_to_sleep < 0) {
+        return;
+    }
+
+    cntfrq = gt_cntfrq_period_ns(arm_cpu);
+    seconds = muldiv64(ticks_to_sleep, cntfrq, NANOSECONDS_PER_SECOND);
+    ticks_to_sleep -= muldiv64(seconds, NANOSECONDS_PER_SECOND, cntfrq);
+    nanos = ticks_to_sleep * cntfrq;
+
+    /*
+     * Don't sleep for less than the time a context switch would take,
+     * so that we can satisfy fast timer requests on the same CPU.
+     * Measurements on M1 show the sweet spot to be ~2ms.
+     */
+    if (!seconds && nanos < (2 * SCALE_MS)) {
+        return;
+    }
+
+    ts = (struct timespec) { seconds, nanos };
+    hvf_wait_for_ipi(cpu, &ts);
+}
+
 static void hvf_sync_vtimer(CPUState *cpu)
 {
     ARMCPU *arm_cpu = ARM_CPU(cpu);
@@ -728,6 +804,9 @@ int hvf_vcpu_exec(CPUState *cpu)
     }
     case EC_WFX_TRAP:
         advance_pc = true;
+        if (!(syndrome & WFX_IS_WFE)) {
+            hvf_wfi(cpu);
+        }
         break;
     case EC_AA64_HVC:
         cpu_synchronize_state(cpu);