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SubjectRe: [PATCH 06/54] x86/kvm: replace bitmap_weight with bitmap_empty where appropriate
Date
Yury Norov <yury.norov@gmail.com> writes:

> In some places kvm/hyperv.c code calls bitmap_weight() to check if any bit
> of a given bitmap is set. It's better to use bitmap_empty() in that case
> because bitmap_empty() stops traversing the bitmap as soon as it finds
> first set bit, while bitmap_weight() counts all bits unconditionally.
>
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
> arch/x86/kvm/hyperv.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 6e38a7d22e97..2c3400dea4b3 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -90,7 +90,7 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
> {
> struct kvm_vcpu *vcpu = hv_synic_to_vcpu(synic);
> struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
> - int auto_eoi_old, auto_eoi_new;
> + bool auto_eoi_old, auto_eoi_new;
>
> if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
> return;
> @@ -100,16 +100,16 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
> else
> __clear_bit(vector, synic->vec_bitmap);
>
> - auto_eoi_old = bitmap_weight(synic->auto_eoi_bitmap, 256);
> + auto_eoi_old = bitmap_empty(synic->auto_eoi_bitmap, 256);

I would've preferred this written as

auto_eoi_old = !bitmap_empty(synic->auto_eoi_bitmap, 256);

so the variable would indicate wether AutoEOI was previosly enabled, not
disabled.

>
> if (synic_has_vector_auto_eoi(synic, vector))
> __set_bit(vector, synic->auto_eoi_bitmap);
> else
> __clear_bit(vector, synic->auto_eoi_bitmap);
>
> - auto_eoi_new = bitmap_weight(synic->auto_eoi_bitmap, 256);
> + auto_eoi_new = bitmap_empty(synic->auto_eoi_bitmap, 256);

Same here, of course. "auto_eoi_new = true" sounds like "AutoEOI is now
enabled".

>
> - if (!!auto_eoi_old == !!auto_eoi_new)
> + if (auto_eoi_old == auto_eoi_new)
> return;
>
> down_write(&vcpu->kvm->arch.apicv_update_lock);

The change look good to me otherwise, feel free to add

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly

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