Re: [PATCH v4 05/19] x86/reboot: Assert that IRQs are disabled when turning off virtualization

[Peter Zijlstra]

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.