linux-kernel - Re: [PATCH 07/49] KVM: x86: replace bitmap_weight with bitmap

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Date:   Fri, 11 Feb 2022 09:47:03 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 07/49] KVM: x86: replace bitmap_weight with bitmap_empty
 where appropriate

On Fri, Feb 11, 2022 at 05:19:36PM +0000, Sean Christopherson wrote:
> On Fri, Feb 11, 2022, Christophe JAILLET wrote:
> > Le 10/02/2022 à 23:48, Yury Norov a écrit :
> > > 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@...il.com>
> > > Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.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..06c2a5603123 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 think that you can also remove the "!" here, ...
> > 
> > >   	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);
> > 
> > ... and there...
> > 
> > > -	if (!!auto_eoi_old == !!auto_eoi_new)
> > > +	if (auto_eoi_old == auto_eoi_new)
> > 
> > ... because this test would still give the same result.

This is how it was in v3. Vitaly asked to add '!' to keep variables
names correct.
https://lore.kernel.org/lkml/CAAH8bW_u6oNOkMsA_jRyWFHkzjMi0CB7gXmvLYAdjNMSqrrY7w@mail.gmail.com/t/#m51d28c03eafed5754a69f95f24c7d0a0510cc5c0
> 
> It would give the same result, but the variable names would be inverted as they
> track if "auto EOI" is being used.  So yes, it's technically unnecessary, but
> also very deliberate.

auto_eoi_old_not_used = bitmap_empty() is worse to me than
auto_eoi_old = !bitmap_empty().

Thanks,
Yury