From: Gleb Natapov <gleb@redhat.com>
Subject: Re: No kernel interface to reset a VCPU
Date: Fri, 25 Sep 2009 20:13:56 +0300
Message-ID: <20090925171356.GB30416@redhat.com>
References: <4ABCD915.5090503@siemens.com>
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Cc: kvm-devel <kvm@vger.kernel.org>
To: Jan Kiszka <jan.kiszka@siemens.com>
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In-Reply-To: <4ABCD915.5090503@siemens.com>
Sender: kvm-owner@vger.kernel.org

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.

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

--
			Gleb.