On Mon, Dec 21, 2009 at 6:22 AM, Alexander Graf <agraf@xxxxxxx> wrote:
> We treated the DEC interrupt like an edge based one. This is not true for
> Book3s. The DEC keeps firing until mtdec is issued again and thus clears
> the interrupt line.
That's not quite right. The decrementer keeps firing until the top bit
is cleared, i.e. with mtdec. However, not *every* mtdec clears it.
(Also, I'm pretty sure this varies between Book 3S implementations,
e.g. 970 behaves differently than POWERn. I don't remember specific
values of <n> though, and I could be misremembering...)
So is this the failure mode?
- a decrementer interrupt is delivered
- guest does *not* issue mtdec to clear it (ppc64's lazy interrupt disabling?)
- guest expects a second decrementer interrupt, but KVM doesn't deliver one
In that case, it seems like the real fix would be something like this:
void kvmppc_emulate_dec(struct kvm_vcpu *vcpu)
{
unsigned long dec_nsec;
pr_debug("mtDEC: %x\n", vcpu->arch.dec);
#ifdef CONFIG_PPC64
/* POWER4+ triggers a dec interrupt if the value is < 0 */
if (vcpu->arch.dec & 0x80000000) {
hrtimer_try_to_cancel(&vcpu->arch.dec_timer);
kvmppc_core_queue_dec(vcpu);
+ /* keep queuing interrupts until guest clears high MSR bit */
+ hrtimer_start(&vcpu->arch.dec_timer, ktime_set(0, 100),
+ HRTIMER_MODE_REL);
return;
}
#endif
-Hollis
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