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Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request

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On Thu, Apr 06, 2017 at 10:20:56PM +0200, Radim Krčmář wrote:
> We want to have kvm_make_all_cpus_request() to be an optmized version of
> 
>   kvm_for_each_vcpu(i, vcpu, kvm) {
>     kvm_make_request(vcpu, request);
>     kvm_vcpu_kick(vcpu);
>   }
> 
> and kvm_vcpu_kick() wakes up the target vcpu.  We know which requests do
> not need the wake up and use it to optimize the loop.

Any reason we don't want kvm_vcpu_kick() to also get the
if (!(req & KVM_REQUEST_NO_WAKEUP)) optimization condition?  I did some
grepping, and don't see any kicks of the requests that have been marked as
NO_WAKEUP, so nothing should change by adding it now.  But the consistency
would be nice for the doc I'm writing.

Also, the condition in kvm_vcpu_kick() looks like overkill

 cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu)

How could vcpu->cpu ever be any offline/invalid cpu, other than -1?  The
condition in kvm_make_all_cpus_request() makes more sense to me

 cpu != -1 && cpu != me

I guess a lot this stuff is planned for a larger requests rework, when
kicks get integrated with requests?  I'm a bit anxious, though, as it
changes how I document stuff now, and even how I approach the ARM series.
For example, if kvm_make_request() already integrated kvm_vcpu_kick(),
which means also adding the smp_mb__after_atomic(), like
kvm_make_all_cpus_request() has, then I wouldn't need to add the smp_mb()
to kvm_arch_vcpu_should_kick().

Thanks,
drew

> 
> Thanks to that, this patch doesn't change the behavior of current users
> (the all don't need the wake up) and only prepares for future where the
> wake up is going to be needed.
> 
> I think that most requests do not need the wake up, so we would flip the
> bit then.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>  virt/kvm/kvm_main.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a486c6ad27a6..1db503bab3dc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -186,6 +186,9 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
>  		/* Set ->requests bit before we read ->mode. */
>  		smp_mb__after_atomic();
>  
> +		if (!(req & KVM_REQUEST_NO_WAKEUP))
> +			kvm_vcpu_wake_up(vcpu);
> +
>  		if (cpus != NULL && cpu != -1 && cpu != me &&
>  				kvm_arch_vcpu_should_kick(vcpu))
>  			cpumask_set_cpu(cpu, cpus);
> -- 
> 2.12.0
> 

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[PATCH 0/6] KVM: towards maintainable kvm_make_all_cpus_request() Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 22:30 +0200
  [PATCH 4/6] KVM: remove #ifndef CONFIG_S390 around kvm_vcpu_wake_up Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 22:30 +0200
    Re: [PATCH 4/6] KVM: remove #ifndef CONFIG_S390 around  kvm_vcpu_wake_up Christian Borntraeger <borntraeger@de.ibm.com> - 2017-04-07 13:10 +0200
  [PATCH RFC 1/6] KVM: fix guest_mode optimization in kvm_make_all_cpus_request() Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 22:30 +0200
    Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() James Hogan <james.hogan@imgtec.com> - 2017-04-06 23:10 +0200
      Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Andrew Jones <drjones@redhat.com> - 2017-04-10 18:00 +0200
        Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() James Hogan <james.hogan@imgtec.com> - 2017-04-11 12:50 +0200
      Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Paolo Bonzini <pbonzini@redhat.com> - 2017-04-11 11:20 +0200
        Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() James Hogan <james.hogan@imgtec.com> - 2017-04-11 11:40 +0200
          Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Radim Krčmář <rkrcmar@redhat.com> - 2017-04-11 21:40 +0200
            Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Paolo Bonzini <pbonzini@redhat.com> - 2017-04-11 21:50 +0200
        Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Radim Krčmář <rkrcmar@redhat.com> - 2017-04-11 22:50 +0200
          Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Paolo Bonzini <pbonzini@redhat.com> - 2017-04-12 02:20 +0200
    Re: [PATCH RFC 1/6] KVM: fix guest_mode optimization in  kvm_make_all_cpus_request() Christian Borntraeger <borntraeger@de.ibm.com> - 2017-04-07 12:50 +0200
  [PATCH 3/6] KVM: x86: use kvm_make_request instead of set_bit Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 22:30 +0200
    Re: [PATCH 3/6] KVM: x86: use kvm_make_request instead of set_bit David Hildenbrand <david@redhat.com> - 2017-04-07 10:20 +0200
  [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 22:30 +0200
    Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request Andrew Jones <drjones@redhat.com> - 2017-04-10 13:20 +0200
      Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request Paolo Bonzini <pbonzini@redhat.com> - 2017-04-11 11:20 +0200
        Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request Andrew Jones <drjones@redhat.com> - 2017-04-11 14:10 +0200
    Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request Paolo Bonzini <pbonzini@redhat.com> - 2017-04-11 11:20 +0200
    Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request Paolo Bonzini <pbonzini@redhat.com> - 2017-04-11 11:20 +0200
  [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of {test,clear}_bit Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 22:30 +0200
    Re: [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of  {test,clear}_bit Christian Borntraeger <borntraeger@de.ibm.com> - 2017-04-07 13:00 +0200
      Re: [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of  {test,clear}_bit Radim Krčmář <rkrcmar@redhat.com> - 2017-04-07 14:30 +0200
        Re: [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of  {test,clear}_bit Radim Krčmář <rkrcmar@redhat.com> - 2017-04-07 16:10 +0200

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