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

[Paolo Bonzini <pbonzini@redhat.com>]

> 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?

Paolo