Re: Question on skip_emulated_instructions()

[Gleb Natapov <gleb@redhat.com>]

On Thu, Apr 08, 2010 at 02:27:53PM +0900, Yoshiaki Tamura wrote:
> Avi Kivity wrote:
> >On 04/07/2010 08:21 PM, Yoshiaki Tamura wrote:
> >>
> >>The problem here is that, I needed to transfer the VM state which is
> >>just *before* the output to the devices. Otherwise, the VM state has
> >>already been proceeded, and after failover, some I/O didn't work as I
> >>expected.
> >>I tracked down this issue, and figured out rip was already proceeded
> >>in KVM,
> >>and transferring this VCPU state was meaningless.
> >>
> >>I'm planning to post the patch set of Kemari soon, but I would like to
> >>solve
> >>this rip issue before that. If there is no drawback, I'm happy to work
> >>and post a patch.
> >
> >vcpu state is undefined when an mmio operation is pending,
> >Documentation/kvm/api.txt says the following:
> >
> >>NOTE: For KVM_EXIT_IO, KVM_EXIT_MMIO and KVM_EXIT_OSI, the corresponding
> >>operations are complete (and guest state is consistent) only after
> >>userspace
> >>has re-entered the kernel with KVM_RUN. The kernel side will first finish
> >>incomplete operations and then check for pending signals. Userspace
> >>can re-enter the guest with an unmasked signal pending to complete
> >>pending operations.
> 
> Thanks for the information.
> 
> So the point is the vcpu state that can been observed from qemu upon
> KVM_EXIT_IO, KVM_EXIT_MMIO and KVM_EXIT_OSI should not be used
> because it's not complete/consistent?
> 
Definitely. VCPU is in the middle of an instruction execution, so the
state is undefined. One instruction may generate more then one IO exit
during its execution BTW.

--
			Gleb.