accel/hvf: Implement WFI without using pselect()

Return to the main loop where we'll be waken again.
This avoid a tricky race with signals introduced in
commit 219c101fa7 ("Add HVF WFI handler").

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Message-ID: <20260112103034.65310-14-philmd@linaro.org>

accel/hvf/hvf-accel-ops.c

@@ -135,8 +135,6 @@ static int hvf_init_vcpu(CPUState *cpu)
     sigaction(SIG_IPI, &sigact, NULL);
 
 #ifdef __aarch64__
-    pthread_sigmask(SIG_BLOCK, NULL, &cpu->accel->unblock_ipi_mask);
-    sigdelset(&cpu->accel->unblock_ipi_mask, SIG_IPI);
     cpu->accel->guest_debug_enabled = false;
 
     r = hv_vcpu_create(&cpu->accel->fd,

include/system/hvf_int.h

@@ -47,7 +47,6 @@ struct AccelCPUState {
 #ifdef __aarch64__
     hv_vcpu_exit_t *exit;
     bool vtimer_masked;
-    sigset_t unblock_ipi_mask;
     bool guest_debug_enabled;
 #endif
 };

target/arm/hvf/hvf.c

@@ -301,7 +301,7 @@ void hvf_arm_init_debug(void)
 #define TMR_CTL_IMASK   (1 << 1)
 #define TMR_CTL_ISTATUS (1 << 2)
 
-static void hvf_wfi(CPUState *cpu);
+static int hvf_wfi(CPUState *cpu);
 
 static uint32_t chosen_ipa_bit_size;
 
@@ -1703,81 +1703,17 @@ static uint64_t hvf_vtimer_val_raw(void)
     return mach_absolute_time() - hvf_state->vtimer_offset;
 }
 
-static uint64_t hvf_vtimer_val(void)
+static int hvf_wfi(CPUState *cpu)
 {
-    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_set_mb(&cpu->thread_kicked, false);
-    bql_unlock();
-    pselect(0, 0, 0, 0, ts, &cpu->accel->unblock_ipi_mask);
-    bql_lock();
-}
-
-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_has_work(cpu)) {
         /*
          * Don't bother to go into our "low power state" if
          * we would just wake up immediately.
          */
-        return;
-    }
-
-    r = hv_vcpu_get_sys_reg(cpu->accel->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->accel->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;
+        return 0;
     }
 
-    ts = (struct timespec) { seconds, nanos };
-    hvf_wait_for_ipi(cpu, &ts);
+    return EXCP_HLT;
 }
 
 /* Must be called by the owning thread */
@@ -1967,7 +1903,7 @@ static int hvf_handle_exception(CPUState *cpu, hv_vcpu_exit_exception_t *excp)
     case EC_WFX_TRAP:
         advance_pc = true;
         if (!(syndrome & WFX_IS_WFE)) {
-            hvf_wfi(cpu);
+            ret = hvf_wfi(cpu);
         }
         break;
     case EC_AA64_HVC: