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Re: [Qemu-devel] [PATCH 1/2] spapr/rtas: disable the decrementer interru
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] spapr/rtas: disable the decrementer interrupt when a CPU is unplugged |
Date: |
Sat, 7 Oct 2017 16:16:50 +1100 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
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 <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.
>
> 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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[Qemu-devel] [PATCH 2/2] spapr/rtas: do not reset the MSR in stop-self command, Cédric Le Goater, 2017/10/05
Re: [Qemu-devel] [PATCH 0/2] disable the decrementer interrupt when a CPU is unplugged, Nikunj A Dadhania, 2017/10/06