Re: [PATCH v3 05/18] x86/reboot: Disable virtualization during reboot iff callback is registered

[Sean Christopherson <seanjc@google.com>]

On Mon, May 22, 2023, Kai Huang wrote:
> On Fri, 2023-05-12 at 16:50 -0700, Sean Christopherson wrote:
> > Attempt to disable virtualization during an emergency reboot if and only
> > if there is a registered virt callback, i.e. iff a hypervisor (KVM) is
> > active.  If there's no active hypervisor, then the CPU can't be operating
> > with VMX or SVM enabled (barring an egregious bug).
> > 
> > Note, IRQs are disabled, which prevents KVM from coming along and enabling
> > virtualization after the fact.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kernel/reboot.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> > index 92b380e199a3..20f7bdabc52e 100644
> > --- a/arch/x86/kernel/reboot.c
> > +++ b/arch/x86/kernel/reboot.c
> > @@ -22,7 +22,6 @@
> >  #include <asm/reboot_fixups.h>
> >  #include <asm/reboot.h>
> >  #include <asm/pci_x86.h>
> > -#include <asm/virtext.h>
> >  #include <asm/cpu.h>
> >  #include <asm/nmi.h>
> >  #include <asm/smp.h>
> > @@ -545,7 +544,7 @@ static void emergency_reboot_disable_virtualization(void)
> >  	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
> >  	 * other CPUs may have virtualization enabled.
> >  	 */
> > -	if (cpu_has_vmx() || cpu_has_svm(NULL)) {
> > +	if (rcu_access_pointer(cpu_emergency_virt_callback)) {
> >  		/* Safely force _this_ CPU out of VMX/SVM operation. */
> >  		cpu_emergency_disable_virtualization();
> 
> 
> IIUC, for cpu_emergency_disable_virtualization() itself, looks it's OK to not
> having the pointer check, since it internally will do rcu_dereference() inside
> RCU critical section anyway.
> 
> But nmi_shootdown_cpus_on_restart() is called after
> cpu_emergency_disable_virtualization(), and having the pointer check here can
> avoid sending NMI to remote cpus if there's no active hypervisor.
> 
> Am I missing something?  If not, is it worth to call this out in changelog?

No, you're not missing anything.  I agree it's worth a line in the changelog.
Dropping the "spurious" NMI should be a-ok, but explicitly calling out the side
effect could be helpful for debug if something is silently relying on the NMI.