Re: [PATCH 2/2] x86/reboot: Disable virtualization in an emergency if SVM is supported

[Sean Christopherson <seanjc@google.com>]

On Thu, May 12, 2022, Thomas Gleixner wrote:
> On Wed, May 11 2022 at 23:43, Sean Christopherson wrote:
> > Disable SVM on all CPUs via NMI shootdown during an emergency reboot.
> > Like VMX, SVM can block INIT and thus prevent bringing up other CPUs via
> > INIT-SIPI-SIPI.
> 
> With the delta patch applied, I'd make that:
> 
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -530,29 +530,25 @@ static inline void kb_wait(void)
>  
>  static inline void nmi_shootdown_cpus_on_restart(void);
>  
> -/* Use NMIs as IPIs to tell all CPUs to disable virtualization */
> -static void emergency_vmx_disable_all(void)
> +static void emergency_reboot_disable_virtualization(void)
>  {
>  	/* Just make sure we won't change CPUs while doing this */
>  	local_irq_disable();
>  
>  	/*
> -	 * Disable VMX on all CPUs before rebooting, otherwise we risk hanging
> -	 * the machine, because the CPU blocks INIT when it's in VMX root.
> +	 * Disable virtualization on all CPUs before rebooting to avoid hanging
> +	 * the system, as VMX and SVM block INIT when running in the host
>  	 *
>  	 * We can't take any locks and we may be on an inconsistent state, so
> -	 * use NMIs as IPIs to tell the other CPUs to exit VMX root and halt.
> +	 * use NMIs as IPIs to tell the other CPUs to disable VMX/SVM and halt.
>  	 *
> -	 * Do the NMI shootdown even if VMX if off on _this_ CPU, as that
> -	 * doesn't prevent a different CPU from being in VMX root operation.
> +	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
> +	 * other CPUs may have virtualization enabled.
>  	 */
> -	if (cpu_has_vmx()) {
> -		/* Safely force _this_ CPU out of VMX root operation. */
> -		__cpu_emergency_vmxoff();
> +	cpu_crash_disable_virtualization();
>  
> -		/* Halt and exit VMX root operation on the other CPUs. */
> +	if (cpu_has_vmx() || cpu_has_svm(NULL))
>  		nmi_shootdown_cpus_on_restart();
> -	}

What about leaving cpu_crash_disable_virtualization() inside the if-statement?
It feels wierd to "disable" virtualization on the current CPU but ignore others,
e.g. if there's some newfangled type of virtualization in the future, I would be
quite surprised if only the CPU doing the transfer needed to disable virtualization.

	if (cpu_has_vmx() || cpu_has_svm(NULL)) {
		/* Safely force _this_ CPU out of VMX/SVM operation. */
		cpu_crash_disable_virtualization();

		/* Disable VMX/SVM and halt on other CPUs. */
		nmi_shootdown_cpus_on_restart()
	}


>  }
>  
>  
> @@ -587,7 +583,7 @@ static void native_machine_emergency_res
>  	unsigned short mode;
>  
>  	if (reboot_emergency)
> -		emergency_vmx_disable_all();
> +		emergency_reboot_disable_virtualization();
>  
>  	tboot_shutdown(TB_SHUTDOWN_REBOOT);
>