Re: kernel device reset support

Avi Kivity
2007-10-09T09:34:38Z

Dong, Eddie wrote:
>>>
>>> +
>>> +/*
>>> + * Reset VM.
>>> + *
>>> + */
>>> +int kvm_vm_reset(struct kvm *kvm)
>>> +{
>>> + struct kvm_vcpu *vcpu;
>>> + int i;
>>> +
>>> + kvm_reset_devices(kvm);
>>> + for (i = 0; i < KVM_MAX_VCPUS; i++) {
>>> + vcpu = kvm->vcpus[i];
>>> + if (!vcpu)
>>> + continue;
>>> + /* active VCPU */
>>> + if (vcpu->vcpu_id)
>>> + vcpu->mp_state = VCPU_MP_STATE_UNINITIALIZED;
>>> + else {
>>> + vcpu->mp_state = VCPU_MP_STATE_RUNNABLE;
>>> + kvm_lapic_reset(vcpu);
>>>
>>>
>> Why is the handling here different?
>>
>
> In native, RESET signal force every processor enter "RESET" status, and
> then immediately after RESET signal is removed, all CPUs will compete
> for BSP role, the winner continue execution, and failor will be blocked
> till INIT/SIPI/SIPI.
>

I meant, you could set both to VCPU_MP_STATE_UNINITIALIZED and let the
vcpu code do the reset differently depending on vcpu_id. This way you
don't run into locking issues (kvm_lapic_reset() needs the vcpu lock?)

>
>>> {
>>> ++vcpu->stat.irq_exits;
>>> + if (vcpu->force_to_quit) {
>>> + vcpu->force_to_quit = 0;
>>> + return -EINTR;
>>> + }
>>> return 1;
>>> }
>>>
>>>
>> Why is this needed?
>>
>
> For a graceful reboot, this one is not needed since every APs are
> already brought to HALT status before BSP issue RESET signal. But in
> case of non-graceful reboot, it is possible the VCPUs are still
> executing guest instruction, so we kick the VCPU and then use this logic
> to force the exception handler to be a heavy VM Exit and execute
> following code at beginning of kvm_vcpu_ioctl_run. (Let
> VCPU_MP_STATE_UNINITIALIZED take effective)
>
> if (unlikely(vcpu->mp_state == VCPU_MP_STATE_UNINITIALIZED)) {
> if (irqchip_in_kernel(vcpu->kvm) && vcpu->apic)
> kvm_lapic_reset(vcpu);
> kvm_vcpu_block(vcpu);
> vcpu_put(vcpu);
> return -EAGAIN;
> }
>
> Whether we need to handle those un-graceful reboot case is up to you :-)
> If not, then those code can be removed.
>

We do need to support ungraceful resets. But this could easily be done
via vcpu->requests.

To reset a vcpu:

set_bit(KVM_REQ_RESET, &vcpu->requests)
kvm_vcpu_kick(vcpu);

And in __vcpu_run():

if (vcpu_requests) {
if (test_and_reset_bit(KVM_REQ_RESET, &vcpu->requests))
....
}

>
>> Userspace can always force an exit by sending a signal.
>>
>>
>>> +/*
>>> + * Reset VM.
>>> + *
>>> + */
>>> +int kvm_vm_reset(struct kvm *kvm)
>>> +{
>>> + struct kvm_vcpu *vcpu;
>>> + int i;
>>> +
>>> + kvm_reset_devices(kvm);
>>>
>>>
>> Need to take kvm->lock around this.
>>
>
> Mmm, I will move this to be after VCPU reset.
> If a VCPU is still accessing (write) devices register, we always have
> problem.
> So move after all the processors enetring frozen state will be simple
> and safer.
>
> Any opnion? Will post after your new comments.
>
>

Sounds good. But the BSP starts executing immediately, no?

So maybe two stages for vcpu reset: first to reset and halt it, then
start it. pic and ioapic reset can be performed in between.

--
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