On Fri, Aug 03, 2012 at 02:41:13PM -0300, Marcelo Tosatti wrote:
> On Fri, Aug 03, 2012 at 06:20:18AM +1000, Benjamin Herrenschmidt wrote:
> > On Thu, 2012-08-02 at 15:35 +0300, Avi Kivity wrote:
> > > This is actually documented in api.txt, though not in relation to
> > > reset:
> > >
> > > 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.
> > >
> > > For x86 the issue was with live migration - you can't copy guest
> > > register state in the middle of an I/O operation. Reset is actually
> > > similar, but it involves writing state (which can then be overwritten)
> > > instead of reading it.
> >
> > Hrm, except that doing KVM_RUN with a signal is very cumbersome to do
> > and I couldn't quite find the logic in qemu to do it ... but I might
> > just have missed it. I can see indeed that in the migration case you
> > want to actually complete the operation rather than just "abort it".
> >
> > Any chance you can point me to the code that performs that trick qemu
> > side for migration ?
>
> kvm-all.c:
>
> kvm_arch_pre_run(env, run);
> if (env->exit_request) {
> DPRINTF("interrupt exit requested\n");
> /*
> * KVM requires us to reenter the kernel after IO exits to
> * complete
> * instruction emulation. This self-signal will ensure that
> * we
> * leave ASAP again.
> */
> qemu_cpu_kick_self();
> }
See kvm_arch_process_async_events() call to qemu_system_reset_request()
in target-i386/kvm.c.
The whole thing is fragile, though: we rely on the order events
are processed inside KVM_RUN, in x86:
1) If there is pending MMIO, process it.
2) If not, return with -EINTR (and KVM_EXIT_INTR) in case
there is a signal pending.
That way, the vcpu will not process the stop event from the main loop
(ie not exit from the kvm_cpu_exec() loop), until MMIO is finished.
> > Anthony seems to think that for reset we can just abort the operation
> > state in the kernel when the MP state changes.
> >
> > Cheers,
> > Ben.
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