Hey Marc,
On Mon, Jul 03, 2023 at 10:45:26AM +0100, Marc Zyngier wrote:
> On Sat, 01 Jul 2023 18:42:28 +0100, Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> > 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...
Heh, I should've mentioned I'm on holiday until Thursday.
> 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.
Yeah, I'm fine with a hack like this. Do you want to send this out as a
patch?
--
Thanks,
Oliver
> 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.
