Jan Kiszka wrote:
>>> Clear the pending original exception when raising a triple fault. This
>>> allows to re-use the vcpu instance, e.g. after a reset which is
>>> typically issued as reaction on the triple fault.
>>>
>>> Signed-off-by: Jan Kiszka <[EMAIL PROTECTED]>
>>>
>>> ---
>>> arch/x86/kvm/x86.c | 4 +++-
>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> Index: b/arch/x86/kvm/x86.c
>>> ===================================================================
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -149,8 +149,10 @@ static void handle_multiple_faults(struc
>>> if (vcpu->arch.exception.nr != DF_VECTOR) {
>>> vcpu->arch.exception.nr = DF_VECTOR;
>>> vcpu->arch.exception.error_code = 0;
>>> - } else
>>> + } else {
>>> set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
>>> + vcpu->arch.exception.pending = false;
>>> + }
>>> }
>>>
>>> void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr)
>>>
>>>
>>>
>> There's a bigger problem here. The exception queue is hidden state that
>> qemu and load and save.
>>
>
> Could you elaborate a bit on what the problematic scenario precisely is
> (that pending triple faults would not be saved/restored while pending
> exceptions are?), and if I/we can do anything to resolve it?
>
Two scenarios:
savevm (no pending exception)
guest runs...
loadvm (with a pending exception in the current state)
spurious exception injected
savevm (pending exception, lost)
new qemu instance (or live migration)
loadvm (exception not delivered)
The second scenario is not too bad, I guess: for fault-like exceptions,
the first instruction would fault again and the exception would be
regenerated. The first scenario is bad, but I guess very unlikely.
One fix would be to expose the exception queue to userspace. I don't
like it since this is not x86 architectural state but a kvm artifact.
Maybe we should clear the exception queue on kvm_set_sregs() (that
should fix the reset case as well).
--
Any sufficiently difficult bug is indistinguishable from a feature.
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