[Xen-devel] [PATCH for-4.5] xen/arm: Fix virtual timer on ARMv8 Model

[Julien Grall <julien.grall@xxxxxxxxxx>]

ARMv8 model may not disable correctly the timer interrupt when Xen
context switch to an idle vCPU. Therefore Xen may receive a spurious
timer interrupt. As the idle domain doesn't have vGIC, Xen will crash
when trying to inject the interrupt with the following stack trace.

(XEN)    [<0000000000228388>] _spin_lock_irqsave+0x28/0x94 (PC)
(XEN)    [<0000000000228380>] _spin_lock_irqsave+0x20/0x94 (LR)
(XEN)    [<0000000000250510>] vgic_vcpu_inject_irq+0x40/0x1b0
(XEN)    [<000000000024bcd0>] vtimer_interrupt+0x4c/0x54
(XEN)    [<0000000000247010>] do_IRQ+0x1a4/0x220
(XEN)    [<0000000000244864>] gic_interrupt+0x50/0xec
(XEN)    [<000000000024fbac>] do_trap_irq+0x20/0x2c
(XEN)    [<0000000000255240>] hyp_irq+0x5c/0x60
(XEN)    [<0000000000241084>] context_switch+0xb8/0xc4
(XEN)    [<000000000022482c>] schedule+0x684/0x6d0
(XEN)    [<000000000022785c>] __do_softirq+0xcc/0xe8
(XEN)    [<00000000002278d4>] do_softirq+0x14/0x1c
(XEN)    [<0000000000240fac>] idle_loop+0x134/0x154
(XEN)    [<000000000024c160>] start_secondary+0x14c/0x15c
(XEN)    [<0000000000000001>] 0000000000000001

While we receive spurious virtual timer interrupt, this could be safely
ignore for the time being. A proper fix need to be found for Xen 4.6.

Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

---

This patch is a bug fix candidate for Xen 4.5. Any ARMv8 model may
randomly crash when running Xen.

This patch don't inject the virtual timer interrupt if the current VCPU
is the idle one. Entering in this function with the idle VCPU is already
a bug itself. For now, I think this patch is the safest way to resolve
the problem.

Meanwhile, I'm investigating with ARM to see wheter the bug comes from
Xen or the model.
---
 xen/arch/arm/time.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
index a6436f1..83c74cb 100644
--- a/xen/arch/arm/time.c
+++ b/xen/arch/arm/time.c
@@ -169,6 +169,14 @@ static void timer_interrupt(int irq, void *dev_id, struct 
cpu_user_regs *regs)
 
 static void vtimer_interrupt(int irq, void *dev_id, struct cpu_user_regs *regs)
 {
+    /*
+     * ARMv8 model may not disable correctly the timer interrupt when
+     * Xen context switch to an idle vCPU. Therefore Xen may receive
+     * timer interrupt.
+     */
+    if ( is_idle_vcpu(current) )
+        return;
+
     current->arch.virt_timer.ctl = READ_SYSREG32(CNTV_CTL_EL0);
     WRITE_SYSREG32(current->arch.virt_timer.ctl | CNTx_CTL_MASK, CNTV_CTL_EL0);
     vgic_vcpu_inject_irq(current, current->arch.virt_timer.irq);
-- 
2.1.3


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