diff mbox series

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

Message ID 20230721201859.2307736-6-seanjc@google.com
State New
Headers
Series x86/reboot: KVM: Clean up "emergency" virt code |

Commit Message

Sean Christopherson July 21, 2023, 8:18 p.m. UTC 
  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(-)

Comments

Peter Zijlstra July 24, 2023, 9:19 p.m. UTC | #1 
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.
Sean Christopherson July 24, 2023, 9:41 p.m. UTC | #2 
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.
diff mbox series

Patch

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)