Re: [PATCH v2 02/18] x86/reboot: Expose VMCS crash hooks if and only if KVM_INTEL is enabled

[Huang, Kai]

Hi Sean,

Thanks for copying me.

On Fri, 2023-03-10 at 13:42 -0800, Sean Christopherson wrote:
> Expose the crash/reboot hooks used by KVM to do VMCLEAR+VMXOFF if and
> only if there's a potential in-tree user, KVM_INTEL.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/reboot.h | 2 ++
>  arch/x86/kernel/reboot.c      | 4 ++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
> index 2551baec927d..33c8e911e0de 100644
> --- a/arch/x86/include/asm/reboot.h
> +++ b/arch/x86/include/asm/reboot.h
> @@ -25,8 +25,10 @@ void __noreturn machine_real_restart(unsigned int type);
>  #define MRR_BIOS	0
>  #define MRR_APM		1
>  
> +#if IS_ENABLED(CONFIG_KVM_INTEL)
>  typedef void crash_vmclear_fn(void);
>  extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
> +#endif
>  void cpu_emergency_disable_virtualization(void);
>  
>  typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> index 299b970e5f82..6c0b1634b884 100644
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -787,6 +787,7 @@ void machine_crash_shutdown(struct pt_regs *regs)
>  }
>  #endif
>  
> +#if IS_ENABLED(CONFIG_KVM_INTEL)
>  /*
>   * This is used to VMCLEAR all VMCSs loaded on the
>   * processor. And when loading kvm_intel module, the
> @@ -807,6 +808,7 @@ static inline void cpu_crash_vmclear_loaded_vmcss(void)
>  		do_vmclear_operation();
>  	rcu_read_unlock();
>  }
> +#endif
>  
>  /* This is the CPU performing the emergency shutdown work. */
>  int crashing_cpu = -1;
> @@ -818,7 +820,9 @@ int crashing_cpu = -1;
>   */
>  void cpu_emergency_disable_virtualization(void)
>  {
> +#if IS_ENABLED(CONFIG_KVM_INTEL)
>  	cpu_crash_vmclear_loaded_vmcss();
> +#endif
>  
>  	cpu_emergency_vmxoff();

In the changelog you mentioned to expose the *hooks* (plural) used to do
"VMCLEAR+VMXOFF" only when KVM_INTEL is on, but here only "VMCLEAR" is embraced
with CONFIG_KVM_INTEL.  So either the changelog needs improvement, or the code
should be adjusted?

Personally, I think it's better to move VMXOFF part within CONFIG_KVM_INTEL too,
if you want to do this.

But I am not sure whether we want to do this (having CONFIG_KVM_INTEL around the
relevant code).  In later patches, you mentioned the case of out-of-tree
hypervisor, for instance, below in the changelog of patch 04:

	There's no need to attempt VMXOFF if KVM (or some other out-of-tree 
	hypervisor) isn't loaded/active...

This means we want to do handle VMCLEAR+VMXOFF in case of out-of-tree hypervisor
too.  So, shouldn't the hooks always exist but not only available when KVM_INTEL
or KVM_AMD is on, so the out-of-tree hypervisor can register their callbacks?


>  	cpu_emergency_svm_disable();
> -- 
> 2.40.0.rc1.284.g88254d51c5-goog
>