[Qemu-ppc] pseries on qemu-system-ppc64le crashes in doorbell_core_ipi()

[Sebastian Andrzej Siewior]

On 2019-12-19 11:41:21 [+0100], Jason A. Donenfeld wrote:
> Hi folks,
Hi,

so this should duct tape it:

diff --git a/arch/powerpc/kernel/dbell.c b/arch/powerpc/kernel/dbell.c
index f17ff1200eaae..ec044bdf362a1 100644
--- a/arch/powerpc/kernel/dbell.c
+++ b/arch/powerpc/kernel/dbell.c
@@ -60,16 +60,8 @@ void doorbell_core_ipi(int cpu)
  */
 int doorbell_try_core_ipi(int cpu)
 {
-	int this_cpu = get_cpu();
 	int ret = 0;
 
-	if (cpumask_test_cpu(cpu, cpu_sibling_mask(this_cpu))) {
-		doorbell_core_ipi(cpu);
-		ret = 1;
-	}
-
-	put_cpu();
-
 	return ret;
 }
 

> This is with "qemu-system-ppc64 -smp 4 -machine pseries" on QEMU 4.0.0.

Interesting. I didn't get v5.4 to boot a while ago and didn't have the
time to look into it.

> I'm not totally sure what's going on here. I'm emulating a pseries, and
> using that with qemu's pseries model, and I see that selecting the pseries
> forces the selection of 'config PPC_DOORBELL' (twice in the same section,
> actually). Then inside the kernel there appears to be some runtime CPU check
> for doorbell support. Is this a case in which QEMU is advertising doorbell
> support that TCG doesn't have? Or is something else happening here?

Based on my understanding is that the doorbell feature is part of the
architecture. It can be used to signal other siblings on the same CPU.
qemu TCG doesn't support that and does not allow to announce multiple
siblings on the same CPU. However, the kernel uses this interface if it
tries to send an interrupt to itself (the same CPU) so everything
matches.
Last time I run into this, the interface was change so the kernel das
not send an IPI to itself. This changed now for another function…

> Thanks,
> Jason

Sebastian