On Fri, Jul 21, 2023 at 01:18:45PM -0700, Sean Christopherson wrote:
> Assert that IRQs are disabled when turning off virtualization in an
> emergency. KVM enables hardware via on_each_cpu(), i.e. could re-enable
> hardware if a pending IPI were delivered after disabling virtualization.
>
> Remove a misleading comment from emergency_reboot_disable_virtualization()
> about "just" needing to guarantee the CPU is stable (see above).
>
> Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kernel/reboot.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> index 48ad2d1ff83d..4cad7183b89e 100644
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -532,7 +532,6 @@ static inline void nmi_shootdown_cpus_on_restart(void);
>
> static void emergency_reboot_disable_virtualization(void)
> {
> - /* Just make sure we won't change CPUs while doing this */
> local_irq_disable();
>
> /*
> @@ -821,6 +820,13 @@ void cpu_emergency_disable_virtualization(void)
> {
> cpu_emergency_virt_cb *callback;
>
> + /*
> + * IRQs must be disabled as KVM enables virtualization in hardware via
> + * function call IPIs, i.e. IRQs need to be disabled to guarantee
> + * virtualization stays disabled.
> + */
> + lockdep_assert_irqs_disabled();
> +
> rcu_read_lock();
> callback = rcu_dereference(cpu_emergency_virt_callback);
> if (callback)
Strictly speaking you don't need rcu_read_lock() when IRQs are already
disabled, but since this is non-performance critical code, it might be
best to keep it super obvious. IOW, carry on.
