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

[David Gibson]

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 <address@hidden>
> ---
>  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.

> +            env->spr[SPR_LPCR] |= LPCR_DEE;
> +        } else {
> +            /* P7 and P8 both have same bit for DECR */
> +            env->spr[SPR_LPCR] |= LPCR_P8_PECE3;
> +        }
> +
>          env->nip = start;
>          env->gpr[3] = r3;
>          cs->halted = 0;
> @@ -210,6 +219,13 @@ static void rtas_stop_self(PowerPCCPU *cpu, 
> sPAPRMachineState *spapr,
>       * no need to bother with specific bits, we just clear it.
>       */
>      env->msr = 0;
> +
> +    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 inline int sysparm_st(target_ulong addr, target_ulong len,

-- 
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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