Re: [PATCH v2 2/9] KVM: x86: remove return value of kvm_vcpu_block

Maxim Levitsky
2022-08-17T14:11:19Z

On Tue, 2022-08-16 at 23:34 +0000, Sean Christopherson wrote:
> On Thu, Aug 11, 2022, Paolo Bonzini wrote:
> > The return value of kvm_vcpu_block will be repurposed soon to return
>
> kvm_vcpu_block()
>
> > the state of KVM_REQ_UNHALT.  In preparation for that, get rid of the
> > current return value.  It is only used by kvm_vcpu_halt to decide whether
>
> kvm_vcpu_halt()
>
> > the call resulted in a wait, but the same effect can be obtained with
> > a single round of polling.
> >
> > No functional change intended, apart from practically indistinguishable
> > changes to the polling behavior.
>
> This "breaks" update_halt_poll_stats().  At the very least, it breaks the comment
> that effectively says "waited" is set if and only if schedule() is called.
>
>         /*
>          * Note, halt-polling is considered successful so long as the vCPU was
>          * never actually scheduled out, i.e. even if the wake event arrived
>          * after of the halt-polling loop itself, but before the full wait.
>          */
>         if (do_halt_poll)
>                 update_halt_poll_stats(vcpu, start, poll_end, !waited);
>
> > @@ -3493,35 +3489,32 @@ void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
> >  {
> >         bool halt_poll_allowed = !kvm_arch_no_poll(vcpu);
> >         bool do_halt_poll = halt_poll_allowed && vcpu->halt_poll_ns;
> > -       ktime_t start, cur, poll_end;
> > +       ktime_t start, cur, poll_end, stop;
> >         bool waited = false;
> >         u64 halt_ns;
> >
> >         start = cur = poll_end = ktime_get();
> > -       if (do_halt_poll) {
> > -               ktime_t stop = ktime_add_ns(start, vcpu->halt_poll_ns);
> > +       stop = do_halt_poll ? start : ktime_add_ns(start, vcpu->halt_poll_ns);
>
> This is inverted, the loop should terminate after the first iteration (stop==start)
> if halt-polling is _not_ allowed, i.e. do_halt_poll is false.
>
> Overall, I don't like this patch.  I don't necessarily hate it, but I definitely
> don't like it.

Roughtly same opinion here.

>
> Isn't freeing up the return from kvm_vcpu_check_block() unnecessary?  Can't we
> just do:
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9f11b505cbee..ccb9f8bdeb18 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10633,7 +10633,7 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
>                 if (hv_timer)
>                         kvm_lapic_switch_to_hv_timer(vcpu);
>
> -               if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
> +               if (!kvm_arch_vcpu_runnable(vcpu))

The 'kvm_vcpu_block()' returns when 'kvm_vcpu_check_block()' returns a negative number
which can happen also due to pending apic timer / signal, however it only sets the
'KVM_REQ_UNHALT' only when 'kvm_arch_vcpu_runnable()==true' so the above does make
sense.

Best regards,
Maxim Levitsky

>                         return 1;
>         }
>
>
> which IMO is more intuitive and doesn't require reworking halt-polling (again).
>
> I don't see why KVM cares if a vCPU becomes runnable after detecting that something
> else caused kvm_vcpu_check_block() to bail.  E.g. a pending signal doesn't invalidate
> a pending vCPU event, and either way KVM is bailing from the run loop.
>