• Christoffer Dall's avatar
    arm/arm64: KVM: Fix migration race in the arch timer · 1a748478
    Christoffer Dall authored
    When a VCPU is no longer running, we currently check to see if it has a
    timer scheduled in the future, and if it does, we schedule a host
    hrtimer to notify is in case the timer expires while the VCPU is still
    not running.  When the hrtimer fires, we mask the guest's timer and
    inject the timer IRQ (still relying on the guest unmasking the time when
    it receives the IRQ).
    
    This is all good and fine, but when migration a VM (checkpoint/restore)
    this introduces a race.  It is unlikely, but possible, for the following
    sequence of events to happen:
    
     1. Userspace stops the VM
     2. Hrtimer for VCPU is scheduled
     3. Userspace checkpoints the VGIC state (no pending timer interrupts)
     4. The hrtimer fires, schedules work in a workqueue
     5. Workqueue function runs, masks the timer and injects timer interrupt
     6. Userspace checkpoints the timer state (timer masked)
    
    At restore time, you end up with a masked timer without any timer
    interrupts and your guest halts never receiving timer interrupts.
    
    Fix this by only kicking the VCPU in the workqueue function, and sample
    the expired state of the timer when entering the guest again and inject
    the interrupt and mask the timer only then.
    Signed-off-by: default avatarChristoffer Dall <christoffer.dall@linaro.org>
    Signed-off-by: default avatarAlex Bennée <alex.bennee@linaro.org>
    Signed-off-by: default avatarChristoffer Dall <christoffer.dall@linaro.org>
    1a748478
arch_timer.c 8.79 KB