Re: [Qemu-devel] [PATCH 1/2] spapr/rtas: disable the decrementer interrupt when a CPU is unplugged

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Oct 06, 2017 at 11:15:31PM +0200, Cédric Le Goater wrote:
> On 10/06/2017 11:07 AM, David Gibson wrote:
> > On Thu, Oct 05, 2017 at 06:49:58PM +0200, Cédric Le Goater wrote:
> >> When a CPU is stopped with the 'stop-self' RTAS call, its state
> >> 'halted' is switched to 1 and, in this case, the MSR is not taken into
> >> account anymore in the cpu_has_work() routine. Only the pending
> >> hardware interrupts are checked with their LPCR:PECE* enablement bit.
> >>
> >> If the DECR timer fires after 'stop-self' is called and before the CPU
> >> 'stop' state is reached, the nearly-dead CPU will have some work to do
> >> and the guest will crash. This case happens very frequently with the
> >> not yet upstream P9 XIVE exploitation mode. In XICS mode, the DECR is
> >> occasionally fired but after 'stop' state, so no work is to be done
> >> and the guest survives.
> >>
> >> I suspect there is a race between the QEMU mainloop triggering the
> >> timers and the TCG CPU thread but I could not quite identify the root
> >> cause. To be safe, let's disable the decrementer interrupt in the LPCR
> >> when the CPU is halted and reenable it when the CPU is restarted.
> >>
> >> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >> ---
> >>  hw/ppc/spapr_rtas.c | 16 ++++++++++++++++
> >>  1 file changed, 16 insertions(+)
> >>
> >> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
> >> index cdf0b607a0a0..2389220c9738 100644
> >> --- a/hw/ppc/spapr_rtas.c
> >> +++ b/hw/ppc/spapr_rtas.c
> >> @@ -174,6 +174,15 @@ static void rtas_start_cpu(PowerPCCPU *cpu_, sPAPRMachineState *spapr,
> >>          kvm_cpu_synchronize_state(cs);
> >>  
> >>          env->msr = (1ULL << MSR_SF) | (1ULL << MSR_ME);
> >> +
> >> +        /* Enable DECR interrupt */
> >> +        if (env->mmu_model == POWERPC_MMU_3_00) {
> > 
> > Hm.  Checking mmu_model doesn't seem right to me.  I mean, it'll get
> > the right answer in practice, but the LPCR programming has nothing
> > whatsoever to do with the MMU.
> > 
> > I think explicitly checking if cpu_ is a POWER9 instance with
> > object_dynamic_cast would be a better option.
> 
> OK. So I guess we should change the switch statement in cpu_ppc_set_papr()
> also.

Yeah, I guess so.  No rush.

-- 
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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