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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