Re: [PATCH v2] KVM: x86: Convert INIT and SIPI signals into synchronously handled requests

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Mar 06, 2013 at 10:39:27PM +0100, Jan Kiszka wrote:
> On 2013-03-06 22:30, Marcelo Tosatti wrote:
> > On Wed, Mar 06, 2013 at 08:57:54AM +0100, Jan Kiszka wrote:
> >> On 2013-03-06 07:12, Paolo Bonzini wrote:
> >>>
> >>>> On Tue, Mar 05, 2013 at 08:16:41PM -0300, Marcelo Tosatti wrote:
> >>>>> On Mon, Mar 04, 2013 at 10:41:43PM +0100, Jan Kiszka wrote:
> >>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>>
> >>>>>> A VCPU sending INIT or SIPI to some other VCPU races for setting
> >>>>>> the
> >>>>>> remote VCPU's mp_state. When we were unlucky,
> >>>>>> KVM_MP_STATE_INIT_RECEIVED
> >>>>>> was overwritten by kvm_emulate_halt and, thus, got lost.
> >>>>>>
> >>>>>> Fix this by raising requests on the sender side that will then be
> >>>>>> handled synchronously over the target VCPU context.
> >>>>>>
> >>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>
> >>>>> Why is kvm_emulate_halt being executed from
> >>>>> KVM_MP_STATE_INIT_RECEIVED/KVM_MP_STATE_SIPI_RECEIVED again?
> >>>>>
> >>>>> Why is it not true that the only valid transition from
> >>>>> KVM_MP_STATE_HALTED is from KVM_MP_STATE_RUNNABLE?
> >>>>
> >>>> See Paolo's table, it is. So why fix a race which should not be
> >>>> happening in the first place.
> >>>
> >>> The bad transition happens exactly because of the race.
> >>> Are you saying you prefer the solution with cmpxchg?
> >>
> >> I think we are past that point in our discussion and should really
> >> separate signal (INIT/SIPI) from state (INIT/SIPI_RECEIVED etc.).
> >>
> >> Jan
> > 
> > The sentence "KVM_MP_STATE_INIT_RECEIVED overwritten by
> > kvm_emulate_halt" is contradictory, unless i miss something.
> 
> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/105638
> 
> Jan

"A VCPU sending INIT or SIPI to some other VCPU races for setting the
remote VCPU's mp_state. When we were unlucky, KVM_MP_STATE_INIT_RECEIVED
was overwritten by kvm_emulate_halt and, thus, got lost.


Fix this by raising requests on the sender side that will then be
handled synchronously over the target VCPU context."


The scenario you describe is:

vcpu0,bsp						vcpu1

vcpu0->mp_state=KVM_MP_STATE_RUNNABLE
							vcpu1->mp_state=KVM_MP_STATE_UNINIT

at __accept_apic_irq()
vcpu1->mp_state=KVM_MP_STATE_INIT_RECEIVED
							kvm_emulate_halt
							vcpu1->mp_state=
							KVM_MP_STATE_HALTED


This is what the first sentence from the patch refers to, correct?