Re: [Qemu-devel] [PATCH] vl: Move cpu_synchronize_all_states() into qemu_system_reset()

[David Gibson]

On Mon, Apr 04, 2016 at 09:31:16AM +0200, Paolo Bonzini wrote:
> 
> 
> On 04/04/2016 06:49, David Gibson wrote:
> > There are currently 3 calls to qemu_system_reset() in vl.c.  Two of them
> > are immediately preceded by a cpu_synchronize_all_states9) and the
> > remaining one should be.
> > 
> > The one which doesn't is the very first reset called directly from main().
> > Without a cpu_synchronize_all_states(), kvm_vcpu_dirty is false at this
> > point from the earlier cpu_synchronize_all_post_init().  That's incorrect
> > because the reset path is quite likely to update the CPU state, and that
> > updated state should be pushed back to KVM, not overwritten with stale
> > data pushed to KVM immediately after init.
> > 
> > This patch moves the call to cpu_synchronize_all_states() into
> > qemu_system_reset() for safety, so it is always called.  AFAICT this should
> > be safe for the handful of callers outside vl.c - these all appear to be in
> > places where the cpu state is already synchronized so the extra call
> > will be a no-op.
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  vl.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > This fixes a real but on ppc - the incorrect state of kvm_vcpu_dirty
> > means that an extra cpu_synchronize_state() in revised code for
> > updating the MSR is not a no-op as expected and loads a stale MSR
> > value, resulting in an incorrect MSR value on entry to the guest.
> > 
> > Therefore, I'm hoping to apply this ASAP to 2.6.
> > 
> > Laurent, could you please verify that this does indeed address the
> > problem with an incorrect MSR on entry.
> > 
> > diff --git a/vl.c b/vl.c
> > index bd81ea9..3629336 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -1745,6 +1745,8 @@ void qemu_system_reset(bool report)
> >  
> >      mc = current_machine ? MACHINE_GET_CLASS(current_machine) : NULL;
> >  
> > +    cpu_synchronize_all_states();
> > +
> >      if (mc && mc->reset) {
> >          mc->reset();
> >      } else {
> > @@ -1893,7 +1895,6 @@ static bool main_loop_should_exit(void)
> >      }
> >      if (qemu_reset_requested()) {
> >          pause_all_vcpus();
> > -        cpu_synchronize_all_states();
> >          qemu_system_reset(VMRESET_REPORT);
> >          resume_all_vcpus();
> >          if (!runstate_check(RUN_STATE_RUNNING) &&
> > @@ -1903,7 +1904,6 @@ static bool main_loop_should_exit(void)
> >      }
> >      if (qemu_wakeup_requested()) {
> >          pause_all_vcpus();
> > -        cpu_synchronize_all_states();
> >          qemu_system_reset(VMRESET_SILENT);
> >          notifier_list_notify(&wakeup_notifiers, &wakeup_reason);
> >          wakeup_reason = QEMU_WAKEUP_REASON_NONE;
> > 
> 
> Acked-by: Paolo Bonzini <address@hidden>

Thanks Paolo.

Paolo / Peter, should I send this in a pull request for my tree, or
should it go through someone else's tree?

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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