On 22/11/17 22:25, Radim Krčmář wrote:
2017-11-21 17:30+0200, Liran Alon:
Do not consider pending exception when return injected exception
to user-mode. A "pending" exception means it's side-effect have not
been applied yet. In contrast, an "injected" exception means it's
side-effect have been already applied.
Therefore, we only need to report of injected exceptions to user-mode.
This is aligned with how interrupts are reported in same ioctl.
Pending interrupts are stored in IRR, but we don't have anything for
exceptions -- we would lose a trap exception that was made pending after
handling inject_pending_event() if the VCPU got a userspace signal and
save+restored before starting the next vcpu_enter_guest() cycle.
(Non-trap exceptions should be generated again when re-executing, so
losing them isn't that bad.)
I see your point. I was indeed not considering correctly what will
happen with trap exceptions.
I think we should add state for pending exceptions in kvm_vcpu_events,
like the FIXME says. Pending and injected are actually exclusive (for
now?), so maybe we can use only one bit for that,
The FIXME is a bit incorrect as it states "This is only needed for
nested virtualization". As I understand this, it is not true as the
issue relates to handling trap exceptions in general. But it is true
that we should return extra state here.
I also observed that pending & injected should be exclusive but didn't
want to change code too much in one shot.
You are right that in order to handle trap exceptions correctly we
should also return to user-space if exception is in pending or injected
state.
I can change struct kvm_vcpu_events.exception such that:
1. "injected" field will remain logical OR of "exception.pending |
.injected".
2. "pad" will become a "flags" field which currently contain a single
flag indicating if exception is pending or not.
That way, I think I don't break any backwards compatibility.
What do you think?
Regards,
-Liran
P.S:
I would be glad if you could review also the rest of the patches in this
series as they are independent of this change and are also a bit
complex. Especially "[PATCH 7/8]: KVM: nVMX: Require immediate-exit when
event reinjected to L2 and L1 event pending".
thanks.
An alternative, probably unattainable, would be to process the
side-effects as we hit the exception. Using IRR to store pending
interrupts also seems possible, but I'd expect more problems down the
road.
