linux-kernel - Re: [PATCH v2 05/18] x86/reboot: KVM: Disable SVM during reboot via virt/KVM reboot callback

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Message-ID: <ZBEVc0/vD5tEj29e@google.com>
Date:   Tue, 14 Mar 2023 17:47:43 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     Chao Gao <chao.gao@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/18] x86/reboot: KVM: Disable SVM during reboot via
 virt/KVM reboot callback

On Tue, Mar 14, 2023, Huang, Kai wrote:
> On Mon, 2023-03-13 at 10:18 -0700, Sean Christopherson wrote:
> > On Mon, Mar 13, 2023, Huang, Kai wrote:
> > > On Fri, 2023-03-10 at 13:42 -0800, Sean Christopherson wrote:
> > > > Use the virt callback to disable SVM (and set GIF=1) during an emergency
> > > > instead of blindly attempting to disable SVM.ï¿½ Like the VMX case, if KVM
> > > > (or an out-of-tree hypervisor) isn't loaded/active, SVM can't be in use.
> > > > 
> > > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > 
> > > [...]
> > > 
> > > > -#if IS_ENABLED(CONFIG_KVM_INTEL)
> > > > +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> > > > ï¿½/* RCU-protected callback to disable virtualization prior to reboot. */
> > > > ï¿½static cpu_emergency_virt_cb __rcu *cpu_emergency_virt_callback;
> > > > ï¿½
> > > > @@ -821,7 +821,7 @@ int crashing_cpu = -1;
> > > > ï¿½ */
> > > > ï¿½void cpu_emergency_disable_virtualization(void)
> > > > ï¿½{
> > > > -#if IS_ENABLED(CONFIG_KVM_INTEL)
> > > > +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
> > > > ï¿½	cpu_emergency_virt_cb *callback;
> > > > ï¿½
> > > > ï¿½	rcu_read_lock();
> > > > @@ -830,8 +830,6 @@ void cpu_emergency_disable_virtualization(void)
> > > > ï¿½		callback();
> > > > ï¿½	rcu_read_unlock();
> > > > ï¿½#endif
> > > > -	/* KVM_AMD doesn't yet utilize the common callback. */
> > > > -	cpu_emergency_svm_disable();
> > > > ï¿½}
> > > 
> > > Shouldn't the callback be always present since you want to consider 'out-of-
> > > tree' hypervisor case?
> > 
> > No?  The kernel doesn't provide any guarantees for out-of-tree code.  I don't have
> > a super strong preference, though I do like the effective documentation the checks
> > provide.  Buy more importantly, my understanding is that the x86 maintainers want
> > to limit the exposure for these types of interfaces, e.g. `git grep IS_ENABLED\(CONFIG_KVM`
> > for a variety of hooks that are enabled iff KVM is enabled in the kernel config.
> 
> How about doing the embracing the code to do the emergency virt callback as the
> last step?

I like that idea, it also makes a few other patches a bit cleaner.

> I like the "cleanup" work in this series regardless whether we should guard the
> emergency virt callback with CONFIG_KVM_INTEL || CONFIG_KVM_AMD.  If we do the
> actual "cleanup" work first, and put the CONFIG_KVM_INTEL || CONFIG_KVM_AMD as
> the last step, it is also easier to review I guess, because it's kinda a
> separate logic independent to the actual "cleanup" work.
> 
> Also, personally I don't particularly like the middle state in patch 04:
> 
>  void cpu_emergency_disable_virtualization(void)
>  {
>  #if IS_ENABLED(CONFIG_KVM_INTEL)
> -	cpu_crash_vmclear_loaded_vmcss();
> -#endif
> +	cpu_emergency_virt_cb *callback;
>  
> -	cpu_emergency_vmxoff();
> +	rcu_read_lock();
> +	callback = rcu_dereference(cpu_emergency_virt_callback);
> +	if (callback)
> +		callback();
> +	rcu_read_unlock();
> +#endif
> +	/* KVM_AMD doesn't yet utilize the common callback. */
>  	cpu_emergency_svm_disable();
>  }
> 
> Which eventually got fixed up in patch 05:
> 
>  void cpu_emergency_disable_virtualization(void)
>  {
> -#if IS_ENABLED(CONFIG_KVM_INTEL)
> +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>  	cpu_emergency_virt_cb *callback;
>  
>  	rcu_read_lock();
> @@ -830,8 +830,6 @@ void cpu_emergency_disable_virtualization(void)
>  		callback();
>  	rcu_read_unlock();
>  #endif
> -	/* KVM_AMD doesn't yet utilize the common callback. */
> -	cpu_emergency_svm_disable();
>  }
>  
> Could we just merge the two patches together? 

I'd prefer not to squash the two.  I agree it's ugly, but I dislike converting
VMX and SVM at the same time.  I'm not totally opposed to moving everything in
one fell swoop, but my preference is to keep them separate.