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