On Wed, Aug 26, 2015 at 09:46:03AM +0300, Pavel Fedin wrote:
> Commit 71760950bf3dc796e5e53ea3300dec724a09f593
> ("arm/arm64: KVM: add a common vgic_queue_irq_to_lr fn") introduced
> vgic_queue_irq_to_lr() function which checks vgic_dist_irq_is_pending()
> before setting LR_STATE_PENDING bit. However, in some cases, the following
> race condition is possible:
> 1. Userland injects an IRQ with level == 1, this ends up in
> vgic_update_irq_pending(), which in turn calls
> vgic_dist_irq_set_pending() for this IRQ.
> 2. vCPU gets kicked. But kernel does not manage to reschedule it quickly
> (!!!)
> 3. Userland quickly resets the IRQ to level == 0. vgic_update_irq_pending()
> in this case will call vgic_dist_irq_clear_pending() and reset the
> pending flag.
only if the guest configured the interrupt as level-triggered, and in
that case it is the expected behavior.
> 4. vCPU finally wakes up. It successfully rolls through through
> __kvm_vgic_flush_hwstate(), which populates vGIC registers. Before the
> aforementioned commit LR_STATE_PENDING bit was set unconditionally, and
> nothing bad happened. However, now vgic_queue_irq_to_lr() does not set
> any state bits on this LR at all, because vgic_dist_irq_is_pending()
> returns zero (it was reset in step 3). Since this is level-sensitive
> IRQ, we end up in LR containing only LR_EOI_INT bit. The guest will not
> get this interrupt.
>
> This patch fixes the problem by bringing back unconditional setting of
> LR_STATE_PENDING bit.
>
> The bug was caught on Cavium ThunderX machine, kernel v4.1.6, running
> qemu "virt" guest, where it affected pl011 driver.
>
> Signed-off-by: Pavel Fedin <[email protected]>
> ---
> virt/kvm/arm/vgic.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> index fdcad86..90d1671 100644
> --- a/virt/kvm/arm/vgic.c
> +++ b/virt/kvm/arm/vgic.c
> @@ -1111,7 +1111,7 @@ static void vgic_queue_irq_to_lr(struct kvm_vcpu *vcpu,
> int irq,
> kvm_debug("Set active, clear distributor: 0x%x\n", vlr.state);
> vgic_irq_clear_active(vcpu, irq);
> vgic_update_state(vcpu->kvm);
> - } else if (vgic_dist_irq_is_pending(vcpu, irq)) {
> + } else {
> vlr.state |= LR_STATE_PENDING;
> kvm_debug("Set pending: 0x%x\n", vlr.state);
> }
> --
> 2.4.4
>
Hmm, there is definitely an inconsistency in the vgic code by getting
here, but this is the wrong fix. I think you want something more along
the lines of the following (completely untested, not even compile
tested):
diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
index 9eb489a..0d1c377 100644
--- a/virt/kvm/arm/vgic.c
+++ b/virt/kvm/arm/vgic.c
@@ -1132,7 +1132,8 @@ static void vgic_queue_irq_to_lr(struct kvm_vcpu *vcpu,
int irq,
kvm_debug("Set active, clear distributor: 0x%x\n", vlr.state);
vgic_irq_clear_active(vcpu, irq);
vgic_update_state(vcpu->kvm);
- } else if (vgic_dist_irq_is_pending(vcpu, irq)) {
+ } else {
+ BUG_ON(!vgic_dist_irq_is_pending(vcpu, irq));
vlr.state |= LR_STATE_PENDING;
kvm_debug("Set pending: 0x%x\n", vlr.state);
}
@@ -1597,8 +1598,11 @@ static int vgic_update_irq_pending(struct kvm *kvm, int
cpuid,
} else {
if (level_triggered) {
vgic_dist_irq_clear_level(vcpu, irq_num);
- if (!vgic_dist_irq_soft_pend(vcpu, irq_num))
+ if (!vgic_dist_irq_soft_pend(vcpu, irq_num)) {
vgic_dist_irq_clear_pending(vcpu, irq_num);
+ vgic_cpu_irq_clear(vcpu, irq_num);
+ __clear_bit(cpuid, dist->irq_pending_on_cpu);
+ }
}
ret = false;
-Christoffer
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