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Re: [Qemu-ppc] [PATCH 0/2] disable the decrementer interrupt when a CPU
From: |
Cédric Le Goater |
Subject: |
Re: [Qemu-ppc] [PATCH 0/2] disable the decrementer interrupt when a CPU is unplugged |
Date: |
Fri, 6 Oct 2017 08:14:13 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 |
On 10/06/2017 08:10 AM, Nikunj A Dadhania wrote:
> Cédric Le Goater <address@hidden> 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.
yes. I was going to restart the thread on the topic.
Let's how these two little patches are discussed. Then we/you can
resend the missing hunk in reset which is needed to perform a TCG
reboot.
Thanks,
C.
> 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
>
- [Qemu-ppc] [PATCH 1/2] spapr/rtas: disable the decrementer interrupt when a CPU is unplugged, (continued)
[Qemu-ppc] [PATCH 2/2] spapr/rtas: do not reset the MSR in stop-self command, Cédric Le Goater, 2017/10/05
Re: [Qemu-ppc] [PATCH 0/2] disable the decrementer interrupt when a CPU is unplugged, Nikunj A Dadhania, 2017/10/06