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

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 24, 2023, Peter Zijlstra wrote:
> 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@intel.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  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.

Ha!  IIRC, I even had a patch to drop the explicit rcu_read_lock(), but decided
I didn't want to tie the use of cpu_emergency_virt_callback to KVM's behavior of
enabling virtualization via IPIs.