Re: [PATCH v5 09/30] KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock

[Sean Christopherson]

On Thu, Sep 22, 2022, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> Because kvm_count_lock unnecessarily complicates the KVM locking convention
> Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock for
> simplicity.  kvm_arch_hardware_enable/disable() callbacks depend on
> non-preemptiblity with the spin lock.  Add preempt_disable/enable()
> around hardware enable/disable callback to keep the assumption.

There's the other "minor" wrinkle that prior to patch 7, "KVM: Rename and move
CPUHP_AP_KVM_STARTING to ONLINE section, kvm_online_cpu() was called with IRQs
disabled and couldn't sleep, i.e. couldn't acquire a mutex.  That's very important
to capture in the changelog.

> Because kvm_suspend() and kvm_resume() is called with interrupt disabled,
> they don't need preempt_disable/enable() pair.
> 
> Opportunistically add some comments on locking.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

...

> @@ -5028,13 +5029,20 @@ static int kvm_online_cpu(unsigned int cpu)
>  	if (kvm_usage_count) {
>  		WARN_ON_ONCE(atomic_read(&hardware_enable_failed));
>  
> +		/*
> +		 * arch callback kvm_arch_hardware_eanble() assumes that

s/eanble/enable

Though even better would be to avoid function names entirely.

> +		 * preemption is disabled for historical reason.  Disable
> +		 * preemption until all arch callbacks are fixed.
> +		 */

Probably better to put this comment above to the WARN_ON_ONCE() in hardware_enable_nolock()
since that's where the oddity and dependency on arch behavior lies.  And then it
can be turned into a FIXME, e.g.

	/*
	 * FIXME: drop the "preemption disabled" requirement here and in the
	 * disable path once all arch code plays nice with preemption.
	 */

> +		preempt_disable();
>  		hardware_enable_nolock(NULL);
> +		preempt_enable();
>  		if (atomic_read(&hardware_enable_failed)) {
>  			atomic_set(&hardware_enable_failed, 0);
>  			ret = -EIO;
>  		}
>  	}
> -	raw_spin_unlock(&kvm_count_lock);
> +	mutex_unlock(&kvm_lock);
>  	return ret;
>  }
>  
> @@ -5042,6 +5050,8 @@ static void hardware_disable_nolock(void *junk)
>  {
>  	int cpu = raw_smp_processor_id();
>  
> +	WARN_ON_ONCE(preemptible());
> +
>  	if (!cpumask_test_cpu(cpu, cpus_hardware_enabled))
>  		return;
>  	cpumask_clear_cpu(cpu, cpus_hardware_enabled);
> @@ -5050,10 +5060,18 @@ static void hardware_disable_nolock(void *junk)
>  
>  static int kvm_offline_cpu(unsigned int cpu)
>  {
> -	raw_spin_lock(&kvm_count_lock);
> -	if (kvm_usage_count)
> +	mutex_lock(&kvm_lock);
> +	if (kvm_usage_count) {
> +		/*
> +		 * arch callback kvm_arch_hardware_disable() assumes that
> +		 * preemption is disabled for historical reason.  Disable
> +		 * preemption until all arch callbacks are fixed.
> +		 */

I vote to drop this comment and instead document everything in the enable FIXME
(see above).

> +		preempt_disable();
>  		hardware_disable_nolock(NULL);
> -	raw_spin_unlock(&kvm_count_lock);
> +		preempt_enable();
> +	}
> +	mutex_unlock(&kvm_lock);
>  	return 0;
>  }

...

> @@ -5708,15 +5728,27 @@ static void kvm_init_debug(void)
>  
>  static int kvm_suspend(void)
>  {
> -	if (kvm_usage_count)
> +	/*
> +	 * The caller ensures that CPU hotplug is disabled by
> +	 * cpu_hotplug_disable() and other CPUs are offlined.  No need for
> +	 * locking.

Disabling CPU hotplug prevents racing with kvm_online_cpu()/kvm_offline_cpu(), but
doesn't prevent racing with hardware_enable_all()/hardware_disable_all(). 

And the lockdep doesn't mesh with the comment, which explains why kvm_lock doesn't
 _need_ to be held, but not why kvm_lock _can't_ be held.

Maybe this?

	/*
	 * Secondary CPUs and CPU hotplug are disabled across the suspend/resume
	 * callbacks, i.e. no need to acquire kvm_lock to ensure the usage count
	 * is stable.  Assert that kvm_lock is not held as a paranoid sanity
	 * check that the system isn't suspended when KVM is enabling hardware.
	 */

> +	 */
> +	lockdep_assert_not_held(&kvm_lock);
> +
> +	if (kvm_usage_count) {
> +		/*
> +		 * Because kvm_suspend() is called with interrupt disabled,  no
> +		 * need to disable preemption.
> +		 */

Add a lockdep and drop the comment, e.g. below the lockdep_assert_not_held(), add

	lockdep_assert_irqs_disabled();

That covers the "why doesn't this disable preemption" _and_ enforces that IRQs are
indeed disabled.

>  		hardware_disable_nolock(NULL);
> +	}
>  	return 0;
>  }
>  
>  static void kvm_resume(void)
>  {
>  	if (kvm_usage_count) {
> -		lockdep_assert_not_held(&kvm_count_lock);
> +		lockdep_assert_not_held(&kvm_lock);
>  		hardware_enable_nolock(NULL);
>  	}
>  }
> -- 
> 2.25.1
>