Re: [PATCH v2 28/43] KVM: VMX: Remove vCPU from PI wakeup list before updating PID.NV

On Thu, Oct 28, 2021, Maxim Levitsky wrote:
> On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> > Remove the vCPU from the wakeup list before updating the notification
> > vector in the posted interrupt post-block helper.  There is no need to
> > wake the current vCPU as it is by definition not blocking.  Practically
> > speaking this is a nop as it only shaves a few meager cycles in the
> > unlikely case that the vCPU was migrated and the previous pCPU gets a
> > wakeup IRQ right before PID.NV is updated.  The real motivation is to
> > allow for more readable code in the future, when post-block is merged
> > with vmx_vcpu_pi_load(), at which point removal from the list will be
> > conditional on the old notification vector.
> > 
> > Opportunistically add comments to document why KVM has a per-CPU spinlock
> > that, at first glance, appears to be taken only on the owning CPU.
> > Explicitly call out that the spinlock must be taken with IRQs disabled, a
> > detail that was "lost" when KVM switched from spin_lock_irqsave() to
> > spin_lock(), with IRQs disabled for the entirety of the relevant path.
> > 
> > Signed-off-by: Sean Christopherson <[email protected]>
> > ---
> >  arch/x86/kvm/vmx/posted_intr.c | 49 +++++++++++++++++++++++-----------
> >  1 file changed, 33 insertions(+), 16 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> > index 2b2206339174..901b7a5f7777 100644
> > --- a/arch/x86/kvm/vmx/posted_intr.c
> > +++ b/arch/x86/kvm/vmx/posted_intr.c
> > @@ -10,10 +10,22 @@
> >  #include "vmx.h"
> >  
> >  /*
> > - * We maintain a per-CPU linked-list of vCPU, so in wakeup_handler() we
> > - * can find which vCPU should be waken up.
> > + * Maintain a per-CPU list of vCPUs that need to be awakened by wakeup_handler()
> Nit: While at it, it would be nice to rename this to pi_wakeup_hanlder() so
> that it can be more easilly found.

Ah, good catch.

> > + * when a WAKEUP_VECTOR interrupted is posted.  vCPUs are added to the list when
> > + * the vCPU is scheduled out and is blocking (e.g. in HLT) with IRQs enabled.
> s/interrupted/interrupt ?
> 
> Isn't that comment incorrect? As I see, the PI hardware is setup to use the WAKEUP_VECTOR
> when vcpu blocks (in pi_pre_block) and then that vcpu is added to the list.
> The pi_wakeup_hanlder just goes over the list and wakes up all vcpus on the lsit.

Doh, yes.  This patch is predicting the future.  The comment becomes correct as of 

  KVM: VMX: Handle PI wakeup shenanigans during vcpu_put/load

but as of this patch the "scheduled out" piece doesn't hold true.
 
> > + * The vCPUs posted interrupt descriptor is updated at the same time to set its
> > + * notification vector to WAKEUP_VECTOR, so that posted interrupt from devices
> > + * wake the target vCPUs.  vCPUs are removed from the list and the notification
> > + * vector is reset when the vCPU is scheduled in.
> >   */
> >  static DEFINE_PER_CPU(struct list_head, blocked_vcpu_on_cpu);
> Also while at it, why not to rename this to 'blocked_vcpu_list'?
> to explain that this is list of blocked vcpus. Its a per-cpu variable
> so 'on_cpu' suffix isn't needed IMHO.

As you noted, addressed in a future patch.

> > +/*
> > + * Protect the per-CPU list with a per-CPU spinlock to handle task migration.
> > + * When a blocking vCPU is awakened _and_ migrated to a different pCPU, the
> > + * ->sched_in() path will need to take the vCPU off the list of the _previous_
> > + * CPU.  IRQs must be disabled when taking this lock, otherwise deadlock will
> > + * occur if a wakeup IRQ arrives and attempts to acquire the lock.
> > + */
> >  static DEFINE_PER_CPU(spinlock_t, blocked_vcpu_on_cpu_lock);
> >  
> >  static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
> > @@ -101,23 +113,28 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
> >  	WARN(pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR,
> >  	     "Wakeup handler not enabled while the vCPU was blocking");
> >  
> > -	dest = cpu_physical_id(vcpu->cpu);
> > -	if (!x2apic_mode)
> > -		dest = (dest << 8) & 0xFF00;
> > -
> > -	do {
> > -		old.control = new.control = READ_ONCE(pi_desc->control);
> > -
> > -		new.ndst = dest;
> > -
> > -		/* set 'NV' to 'notification vector' */
> > -		new.nv = POSTED_INTR_VECTOR;
> > -	} while (cmpxchg64(&pi_desc->control, old.control,
> > -			   new.control) != old.control);
> > -
> > +	/*
> > +	 * Remove the vCPU from the wakeup list of the _previous_ pCPU, which
> > +	 * will not be the same as the current pCPU if the task was migrated.
> > +	 */
> >  	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> >  	list_del(&vcpu->blocked_vcpu_list);
> >  	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> > +
> > +	dest = cpu_physical_id(vcpu->cpu);
> > +	if (!x2apic_mode)
> > +		dest = (dest << 8) & 0xFF00;
> It would be nice to have a function for this, this appears in this file twice.
> Maybe there is a function already somewhere?

The second instance does go away by the aforementioned:

  KVM: VMX: Handle PI wakeup shenanigans during vcpu_put/load

I'm inclined to say we don't want a helper because there should only ever be one
path that changes PI.ndst.  But a comment would definitely help to explain the
difference between xAPIC and x2APIC IDs.