linux-kernel - Re: [PATCH v2 02/18] x86/reboot: Expose VMCS crash hooks if and only if KVM

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Date:   Mon, 13 Mar 2023 11:31:02 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chao Gao <chao.gao@...el.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 02/18] x86/reboot: Expose VMCS crash hooks if and only
 if KVM_INTEL is enabled

On Mon, Mar 13, 2023, Huang, Kai wrote:
> 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@...gle.com>
> > ---

...

> > 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?

I'll reword the changelog, "hooks" in my head was referring to the regsiter and
unregister "hooks", not the callback itself.

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

That happens eventually in the final third of this series.

> 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-treeï¿½
> 	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?

Ah, I see how I confused things with that statement.  My intent was only to call
out that, technically, a non-NULL callback doesn't mean KVM is loaded.  I didn't
intend to sign the kernel up for going out of its way to support out-of-tree hypervisors.

Does it read better if I add a "that piggybacked the callback" qualifier?

  There's no need to attempt VMXOFF if KVM (or some other out-of-tree hypervisor
  that piggybacked the callback) isn't loaded/active, i.e. if the CPU can't
  possibly be post-VMXON.