Re: [PATCH] cpu: Add starts_halted() method

[Eduardo Habkost]

On Wed, Jul 08, 2020 at 09:11:55PM +0100, Peter Maydell wrote:
> On Wed, 8 Jul 2020 at 18:36, Eduardo Habkost <ehabkost@redhat.com> wrote:
> >
> > On Wed, Jul 08, 2020 at 06:09:49PM +0100, Peter Maydell wrote:
> > > Exactly. It appears that there's a bug in our mechanisms,
> > > which is why I'm suggesting that the right thing is
> > > to fix that bug rather than marking the CPU as halted
> > > earlier in the reset process so that the KVM_RUN happens
> > > to do nothing...
> >
> > I agree this is necessary, but it doesn't seem sufficient.
> >
> > Having cpu_reset() set halted=0 on spapr (and probably other
> > machines) is also a bug, as it could still trigger unwanted
> > KVM_RUN when cpu_reset() returns (and before machine code sets
> > halted=1).
> 
> The Arm handling of starting-halted sets halted=1 within cpu_reset,
> based on whether the CPU object was created with a
> "start-powered-off" property.

Making this mechanism generic sounds like a good idea.

> 
> I'm not sure in practice that anything can get in asynchronously
> and cause a KVM_RUN in between spapr_reset_vcpu() calling
> cpu_reset() and it setting cs->halted (and the other stuff),
> though. This function ought to be called with the iothread
> lock held, so KVM_RUN will only happen if it calls some
> other function which incorrectly lets the CPU run.

Yeah, maybe it won't happen in practice.  It just seems fragile.
The same way ppc_cpu_reset() kicked the CPU by accident, code
outside cpu_reset() might one day kick the CPU by accident before
setting halted=1.

-- 
Eduardo