Re: [Qemu-devel] [PATCH 0/2] disable the decrementer interrupt when a CPU is unplugged

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2017-10-06 at 11:40 +0530, Nikunj A Dadhania wrote:
> Cédric Le Goater <clg@kaod.org> writes:
> 
> > Hello,
> > 
> > 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.
> 
> Moreover, disabling the DECR in the reset path solves the TCG multi cpu
> reboot case, as reboot path does not call stop-cpu rtas call.

SHouldn't we do it in set_papr too and only turn it on for the boot CPU
and in start-cpu RTAS call ? Same with the other PECEs in fact...

> diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> index 3e20b1d886..c5150ee590 100644
> --- a/hw/ppc/spapr_cpu_core.c
> +++ b/hw/ppc/spapr_cpu_core.c
> @@ -86,6 +86,15 @@ static void spapr_cpu_reset(void *opaque)
>      cs->halted = 1;
>  
>      env->spr[SPR_HIOR] = 0;
> +    /* Disable DECR for secondary cpus */
> +    if (cs != first_cpu) {
> +        if (env->mmu_model == POWERPC_MMU_3_00) {
> +            env->spr[SPR_LPCR] &= ~LPCR_DEE;
> +        } else {
> +            /* P7 and P8 both have same bit for DECR */
> +            env->spr[SPR_LPCR] &= ~LPCR_P8_PECE3;
> +        }
> +    }
>  }
>  
>  static void spapr_cpu_destroy(PowerPCCPU *cpu)
> 
> 
> Regards
> Nikunj