spapr/rtas: disable the decrementer interrupt when a CPU is unplugged

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 in the LPCR all the exceptions which
can cause an exit while the CPU is in power-saving mode and reenable
them when the CPU is started.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 11 insertions, 0 deletions

diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
index cdf0b60..858adb1 100644
--- a/hw/ppc/spapr_rtas.c
+++ b/hw/ppc/spapr_rtas.c
@@ -162,6 +162,7 @@ static void rtas_start_cpu(PowerPCCPU *cpu_, sPAPRMachineState *spapr,
     if (cpu != NULL) {
         CPUState *cs = CPU(cpu);
         CPUPPCState *env = &cpu->env;
+        PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
 
         if (!cs->halted) {
             rtas_st(rets, 0, RTAS_OUT_HW_ERROR);
@@ -174,6 +175,10 @@ static void rtas_start_cpu(PowerPCCPU *cpu_, sPAPRMachineState *spapr,
         kvm_cpu_synchronize_state(cs);
 
         env->msr = (1ULL << MSR_SF) | (1ULL << MSR_ME);
+
+        /* Enable Power-saving mode Exit Cause exceptions for the new CPU */
+        env->spr[SPR_LPCR] |= pcc->lpcr_pm;
+
         env->nip = start;
         env->gpr[3] = r3;
         cs->halted = 0;
@@ -197,6 +202,7 @@ static void rtas_stop_self(PowerPCCPU *cpu, sPAPRMachineState *spapr,
 {
     CPUState *cs = CPU(cpu);
     CPUPPCState *env = &cpu->env;
+    PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
 
     cs->halted = 1;
     qemu_cpu_kick(cs);
@@ -210,6 +216,11 @@ static void rtas_stop_self(PowerPCCPU *cpu, sPAPRMachineState *spapr,
      * no need to bother with specific bits, we just clear it.
      */
     env->msr = 0;
+
+    /* Disable Power-saving mode Exit Cause exceptions for the CPU.
+     * This could deliver an interrupt on a dying CPU and crash the
+     * guest */
+    env->spr[SPR_LPCR] &= ~pcc->lpcr_pm;
 }
 
 static inline int sysparm_st(target_ulong addr, target_ulong len,