Re: No kernel interface to reset a VCPU

[Jan Kiszka <jan.kiszka@web.de>]

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Gleb Natapov wrote:
> On Fri, Sep 25, 2009 at 04:52:05PM +0200, Jan Kiszka wrote:
>> Hi all,
>>
>> looks to me like there is no way to properly reset the boot processor.
>> The current pattern used by qemu[-kvm] is to reload all registers with
>> their reset values. But that does not affect the internal VCPU states
>> the KVM keeps in the kernel. They are only reset during VCPU setup or
>> after receiving a SIPI (and the latter only helps with secondary CPUs).
>>
> Userspace should have access to internal VCPU states too, otherwise
> migration will not work.

Good point.

> 
>> So the only way around it with the current kernel interface is to
>> destroy/recreate the BSP on reset, right? /me is looking into such an
>> approach now.
> I don't think destroying/recreating vcpu will work. I don't remember 
> exact details though.
> 
>> We stumbled over inconsistent VCPU states with our internal qemu-kvm
>> tree. We have a legacy watchdog emulation here that triggered but failed
>> to bring up the system again. I wasn't able to create a similar case
>> with a standard environment yet, but I think it is not unrealistic for
>> qemu-kvm as well. Hacking kvm_arch_vcpu_reset() into KVM that triggers
>> on the right register values "solved" the issue here.
>>
> Can you find the root cause of the problem? As I said above qemu should
> have full access to vcpu state for proper migration support. Not that

I just had a closer look again and found our problem: arch.nmi_pending.

I think the risk to be bitten by this on standard OSes is rather low.
The reset issue we see is widely related to the special NMI-based
watchdog here. The probability to see the pattern NMI-> guest handler ->
NMI -> system-reset on ordinary systems is fairly low. Besides this
hidden state may cause lost NMI events during migration or save/restore.
Again a corner case.

But it should be fixed. Will check where we could add this bit for
userland read-out.

> kvm_vcpu_reset()/kvm_apic_reset()/kvm_ioapic_reset()/kvm_pit_reset() is
> bad idea. Actually I want to add them one day.
> 
> --
> 			Gleb.

Jan


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