Re: [PATCH 5/5] arm/arm64: KVM: Initialize the vgic on-demand when injecting IRQs

Christoffer Dall
2014-12-12T20:24:01Z

On Fri, Dec 12, 2014 at 12:37:52PM +0100, Christoffer Dall wrote:
> On Fri, Dec 12, 2014 at 11:23:35AM +0000, Marc Zyngier wrote:
> > On 12/12/14 11:14, Christoffer Dall wrote:
> > > On Thu, Dec 11, 2014 at 06:35:40PM +0000, Marc Zyngier wrote:
> > >> On 09/12/14 15:44, Christoffer Dall wrote:
> > >>> Userspace assumes that it can wire up IRQ injections after having
> > >>> created all VCPUs and after having created the VGIC, but potentially
> > >>> before starting the first VCPU. This can currently lead to lost IRQs
> > >>> because the state of that IRQ injection is not stored anywhere and we
> > >>> don't return an error to userspace.
> > >>>
> > >>> We haven't seen this problem manifest itself yet, presumably because
> > >>> guests reset the devices on boot, but this could cause issues with
> > >>> migration and other non-standard startup configurations.
> > >>>
> > >>> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> > >>> ---
> > >>> virt/kvm/arm/vgic.c | 9 +++++++--
> > >>> 1 file changed, 7 insertions(+), 2 deletions(-)
> > >>>
> > >>> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> > >>> index c98cc6b..feef015 100644
> > >>> --- a/virt/kvm/arm/vgic.c
> > >>> +++ b/virt/kvm/arm/vgic.c
> > >>> @@ -1693,8 +1693,13 @@ out:
> > >>> int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int irq_num,
> > >>> bool level)
> > >>> {
> > >>> - if (likely(vgic_ready(kvm)) &&
> > >>> - vgic_update_irq_pending(kvm, cpuid, irq_num, level))
> > >>> + if (unlikely(!vgic_initialized(kvm))) {
> > >>> + mutex_lock(&kvm->lock);
> > >>> + vgic_init(kvm);
> > >>
> > >> What if this fails?
> > >>
> > > yeah, not good. The thing is that we also don't check the return value
> > > from kvm_vgic_inject_irq(), so we can do two things:
> > >
> > > (1) change this function to a void, carry out the check for
> > > vgic_initialized in kvm_vm_ioctl_irq_line() in arm.c and export
> > > vgic_init() outside of vgic.c.
> > >
> > > (2) just error out if vgic_init() fails and print a kernel error (or
> > > even a BUG_ON?) in kvm_timer_inject_irq() in arch_timer.c.
> > >
> > > In both cases we need to make sure that we never configure the timer to
> > > begin injecting IRQs before the vgic is initialized, as Eric pointed out
> > > before.
> > >
> > > What do you think?
> >
> > I'd favour option two.
> >
> > My reasoning is that the timer interrupt is triggered by the HW. If it
> > has fired, that's because we've programmed it to trigger, with means a
> > vcpu has run. At that point, the vgic would better be initialized, or we
> > have something much nastier on our hands.
> >
> Sounds reasonable to me, I'll do a quick respin with the check for the
> timer (to ensure the user even created a vgic).
>
Just to double-check, it is going to look something like this for the
arch-timer path:

diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
index d4da244..c61d51d 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -441,6 +441,16 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
return ret;
}

+#ifdef CONFIG_KVM_ARM_TIMER
+ /*
+ * If the Architected Timers are supported, userspace must have
+ * created an in-kernel irqchip, since otherwise we will receive
+ * virtual timer interrupt and have nowhere to route them to.
+ */
+ if (!irqchip_in_kernel(kvm))
+ return -ENODEV;
+#endif
+
return 0;
}

diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 22fa819..b10e495 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -61,12 +61,14 @@ static void timer_disarm(struct arch_timer_cpu *timer)

static void kvm_timer_inject_irq(struct kvm_vcpu *vcpu)
{
+ int ret;
struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;

timer->cntv_ctl |= ARCH_TIMER_CTRL_IT_MASK;
- kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
- timer->irq->irq,
- timer->irq->level);
+ ret = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
+ timer->irq->irq,
+ timer->irq->level);
+ BUG_ON(ret);
}

static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
--

Does this look reasonable to you?

-Christoffer