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Re: [Xen-devel] [RFC PATCH 17/49] ARM: timer: Handle level triggered IRQs correctly

Hi Andre,

On 09/02/18 14:39, Andre Przywara wrote:
The ARM Generic Timer uses a level-sensitive interrupt semantic. We
easily catch when the line goes high, as this triggers the hardware IRQ.
However we have to sync the state of the interrupt condition at certain
points to catch when the line goes low and we can remove the vtimer vIRQ
from the vGIC (and the LR).
The VGIC in Xen so far only implemented edge triggered vIRQs, really, so
we need to add new functionality to re-sample the interrupt state.

You might want to make a summary of the discussion we had with Marc Z. today here. This would help the other to understand why sample the interrupt state is necessary :).

Also do we need to do that for the emulated physical timer?

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>
  xen/arch/arm/time.c     | 34 ++++++++++++++++++++++++++++++++++
  xen/arch/arm/traps.c    |  1 +
  xen/include/xen/timer.h |  2 ++
  3 files changed, 37 insertions(+)

diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
index c11fcfeadd..98ebb4305d 100644
--- a/xen/arch/arm/time.c
+++ b/xen/arch/arm/time.c
@@ -263,6 +263,40 @@ static void vtimer_interrupt(int irq, void *dev_id, struct 
cpu_user_regs *regs)
      vgic_inject_irq(current->domain, current, current->arch.virt_timer.irq, 

One * is enough.

+ * vtimer_sync() - update the state of the virtual timer after a guest run
+ * @vcpu: The VCPU to sync the arch timer state
+ *
+ * After returning from a guest, update the state of the virtual interrupt
+ * line, to model the level triggered interrupt correctly.
+ * If the guest has handled a timer interrupt, the virtual interrupt line
+ * needs to be lowered explicitly. vgic_inject_irq() takes care of that.
+ */
+void vtimer_sync(struct vcpu *vcpu)
+    struct vtimer *vtimer = &vcpu->arch.virt_timer;
+    bool level;
+    vtimer->ctl = READ_SYSREG32(CNTV_CTL_EL0);
+    vtimer->cval = READ_SYSREG64(CNTV_CVAL_EL0);

Why do you need to save cval?

+    /*
+     * Technically we should mask with 0x7 here, to catch if the timer
+     * interrupt is masked. However Xen always masks the timer upon entering
+     * the hypervisor, leaving it up to the guest to un-mask it.
+     * So we would always read a "low" level, despite the condition being
+     * actually "high". Igoring the mask bit solves this (for now).


+     * Another possible check would be to compare the value of CNTVCT_EL0
+     * against vtimer->cval and derive the interrupt state from that.
+     *
+     * TODO: The proper fix for this is to make vtimer vIRQ hardware mapped,
+     * but this requires reworking the arch timer to implement this.

That something we should look at it once the vGIC is done :).

+     */
+    level = (vtimer->ctl & 0x5) == (CNTx_CTL_ENABLE | CNTx_CTL_PENDING);

Can you please use the proper define rather than plain value?

+    vgic_inject_irq(vcpu->domain, vcpu, vtimer->irq, level);
   * Arch timer interrupt really ought to be level triggered, since the
   * design of the timer/comparator mechanism is based around that
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 1cba7e584d..2d770a14a5 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -2024,6 +2024,7 @@ static void enter_hypervisor_head(struct cpu_user_regs 
          if ( current->arch.hcr_el2 & HCR_VA )
              current->arch.hcr_el2 = READ_SYSREG(HCR_EL2);

You need to sample the virtual timer before clearing the LRs, right? If so, you likely want to add a comment here to avoid reshuffling the code.

+        vtimer_sync(current);

I am a bit worry about re-sampling the virtual interrupt state at every traps. It might be worth thinking to do the re-sample when syncing the LRs (as you do for HW level interrupt in patch #25). Probably once we get the new vGIC merged.

diff --git a/xen/include/xen/timer.h b/xen/include/xen/timer.h
index 4513260b0d..eddbbf3903 100644
--- a/xen/include/xen/timer.h
+++ b/xen/include/xen/timer.h
@@ -94,6 +94,8 @@ DECLARE_PER_CPU(s_time_t, timer_deadline);
  /* Arch-defined function to reprogram timer hardware for new deadline. */
  int reprogram_timer(s_time_t timeout);
+void vtimer_sync(struct vcpu *vcpu);
  /* Calculate the aligned first tick time for a given periodic timer. */
  s_time_t align_timer(s_time_t firsttick, uint64_t period);


Julien Grall

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