Re: [PATCH v3 10/22] KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock

[Marc Zyngier <maz@kernel.org>]

On Tue, 06 Sep 2022 03:46:43 +0100,
Yuan Yao <yuan.yao@linux.intel.com> wrote:
> 
> On Thu, Sep 01, 2022 at 07:17:45PM -0700, isaku.yamahata@intel.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@intel.com>
> >
> > 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.
> >
> > Opportunistically add some comments on locking.
> >
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > ---
> >  Documentation/virt/kvm/locking.rst | 14 +++++-------
> >  virt/kvm/kvm_main.c                | 34 ++++++++++++++++++++----------
> >  2 files changed, 28 insertions(+), 20 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
> > index 845a561629f1..8957e32aa724 100644
> > --- a/Documentation/virt/kvm/locking.rst
> > +++ b/Documentation/virt/kvm/locking.rst
> > @@ -216,15 +216,11 @@ time it will be set using the Dirty tracking mechanism described above.
> >  :Type:		mutex
> >  :Arch:		any
> >  :Protects:	- vm_list
> > -
> > -``kvm_count_lock``
> > -^^^^^^^^^^^^^^^^^^
> > -
> > -:Type:		raw_spinlock_t
> > -:Arch:		any
> > -:Protects:	- hardware virtualization enable/disable
> > -:Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
> > -		migration.
> > +                - kvm_usage_count
> > +                - hardware virtualization enable/disable
> > +:Comment:	Use cpus_read_lock() for hardware virtualization enable/disable
> > +                because hardware enabling/disabling must be atomic /wrt
> > +                migration.  The lock order is cpus lock => kvm_lock.
> >
> >  ``kvm->mn_invalidate_lock``
> >  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index fc55447c4dba..082d5dbc8d7f 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -100,7 +100,6 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
> >   */
> >
> >  DEFINE_MUTEX(kvm_lock);
> > -static DEFINE_RAW_SPINLOCK(kvm_count_lock);
> >  LIST_HEAD(vm_list);
> >
> >  static cpumask_var_t cpus_hardware_enabled;
> > @@ -4996,6 +4995,8 @@ static void hardware_enable_nolock(void *caller_name)
> >  	int cpu = raw_smp_processor_id();
> >  	int r;
> >
> > +	WARN_ON_ONCE(preemptible());
> 
> This looks incorrect, it may triggers everytime when online CPU.
> Because patch 7 moved CPUHP_AP_KVM_STARTING *AFTER*
> CPUHP_AP_ONLINE_IDLE as CPUHP_AP_KVM_ONLINE, then cpuhp_thread_fun()
> runs the new CPUHP_AP_KVM_ONLINE in *non-atomic* context:
> 
> cpuhp_thread_fun(unsigned int cpu) {
> ...
> 	if (cpuhp_is_atomic_state(state)) {
> 		local_irq_disable();
> 		st->result = cpuhp_invoke_callback(cpu, state, bringup, st->node, &st->last);
> 		local_irq_enable();
> 
> 		WARN_ON_ONCE(st->result);
> 	} else {
> 		st->result = cpuhp_invoke_callback(cpu, state, bringup, st->node, &st->last);
> 	}
> ...
> }
> 
> static bool cpuhp_is_atomic_state(enum cpuhp_state state)
> {
> 	return CPUHP_AP_IDLE_DEAD <= state && state < CPUHP_AP_ONLINE;
> }
> 
> The hardware_enable_nolock() now is called in 2 cases:
> 1. in atomic context by on_each_cpu().
> 2. From non-atomic context by CPU hotplug thread.
> 
> so how about "WARN_ONCE(preemptible() && cpu_active(cpu))" ?

I suspect similar changes must be applied to the arm64 side (though
I'm still looking for a good definition of cpu_active()).

	M.

-- 
Without deviation from the norm, progress is not possible.