On Sat, 01 Jul 2023 18:42:28 +0100,
Oliver Upton <oliver.upton@xxxxxxxxx> 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;
}
--
Without deviation from the norm, progress is not possible.
