KVM CPU hotplug notifier triggers BUG_ON on arm64

[Marc Zyngier]

On Sat, 01 Jul 2023 18:42:28 +0100,
Oliver Upton <oliver.upton at linux.dev> wrote:
> 
> Hi Kristina,
> 
> Thanks for the bug report.
> 
> On Sat, Jul 01, 2023 at 01:50:52PM +0100, Kristina Martsenko wrote:
> > Hi,
> > 
> > When I try to online a CPU on arm64 while a KVM guest is running, I hit a
> > BUG_ON(preemptible()) (as well as a WARN_ON). See below for the full log.
> > 
> > This is on kvmarm/next, but seems to have been broken since 6.3. Bisecting it
> > points at commit:
> > 
> >   0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock")
> 
> Makes sense. We were using a spinlock before, which implictly disables
> preemption.
> 
> Well, one way to hack around the problem would be to just cram
> preempt_{disable,enable}() into kvm_arch_hardware_disable(), but that's
> kinda gross in the context of cpuhp which isn't migratable in the first
> place. Let me have a look...

An alternative would be to replace the preemptible() checks with a one
that looks at the migration state, but I'm not sure that's much better
(it certainly looks more costly).

There is also the fact that most of our per-CPU accessors are already
using preemption disabling, and this code has a bunch of them. So I'm
not sure there is a lot to be gained from not disabling preemption
upfront.

Anyway, as I was able to reproduce the issue under NV, I tested the
hack below. If anything, I expect it to be a reasonable fix for
6.3/6.4, and until we come up with a better approach.

Thanks,

	M.

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index aaeae1145359..a28c4ffe4932 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1894,8 +1894,17 @@ static void _kvm_arch_hardware_enable(void *discard)
 
 int kvm_arch_hardware_enable(void)
 {
-	int was_enabled = __this_cpu_read(kvm_arm_hardware_enabled);
+	int was_enabled;
 
+	/*
+	 * Most calls to this function are made with migration
+	 * disabled, but not with preemption disabled. The former is
+	 * enough to ensure correctness, but most of the helpers
+	 * expect the later and will throw a tantrum otherwise.
+	 */
+	preempt_disable();
+
+	was_enabled = __this_cpu_read(kvm_arm_hardware_enabled);
 	_kvm_arch_hardware_enable(NULL);
 
 	if (!was_enabled) {
@@ -1903,6 +1912,8 @@ int kvm_arch_hardware_enable(void)
 		kvm_timer_cpu_up();
 	}
 
+	preempt_enable();
+
 	return 0;
 }
 



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