On Mon, Jul 03, 2023, Marc Zyngier wrote:
> Since 0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect
> kvm_usage_count with kvm_lock"), hotplugging back a CPU whilst
> a guest is running results in a number of ugly splats as most
> of this code expects to run with preemption disabled, which isn't
> the case anymore.
>
> While the context is preemptable, it isn't migratable, which should
> be enough. But we have plenty of preemptible() checks all over
> the place, and our per-CPU accessors also disable preemption.
>
> Since this affects released versions, let's do the easy fix first,
> disabling preemption in kvm_arch_hardware_enable(). We can always
> revisit this with a more invasive fix in the future.
>
> Fixes: 0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock")
> Reported-by: Kristina Martsenko <kristina.martsenko@xxxxxxx>
> Tested-by: Kristina Martsenko <kristina.martsenko@xxxxxxx>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/aeab7562-2d39-e78e-93b1-4711f8cc3fa5@xxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx # v6.3, v6.4
> ---
> arch/arm64/kvm/arm.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> 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);
IMO, this_cpu_has_cap() is at fault. E.g. why not do this?
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 7d7128c65161..b862477de2ce 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -3193,7 +3193,9 @@ static void __init setup_boot_cpu_capabilities(void)
bool this_cpu_has_cap(unsigned int n)
{
- if (!WARN_ON(preemptible()) && n < ARM64_NCAPS) {
+ __this_cpu_preempt_check("has_cap");
+
+ if (n < ARM64_NCAPS) {
const struct arm64_cpu_capabilities *cap = cpu_hwcaps_ptrs[n];
if (cap)
