linux-kernel - Re: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup

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Message-ID: <CALMp9eS47Gip-yZq0tP5s8xRJ-fcbp41O7n2tBc=8Q-urZ3E6A@mail.gmail.com>
Date:   Wed, 8 Jan 2020 09:50:55 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup

On Wed, Jan 8, 2020 at 2:13 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
>
> > Use a Logical OR in __is_rsvd_bits_set() to combine the two reserved bit
> > checks, which are obviously intended to be logical statements.  Switching
> > to a Logical OR is functionally a nop, but allows the compiler to better
> > optimize the checks.
> >
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 7269130ea5e2..72e845709027 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3970,7 +3970,7 @@ __is_rsvd_bits_set(struct rsvd_bits_validate *rsvd_check, u64 pte, int level)
> >  {
> >       int bit7 = (pte >> 7) & 1, low6 = pte & 0x3f;
> >
> > -     return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) |
> > +     return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) ||
> >               ((rsvd_check->bad_mt_xwr & (1ull << low6)) != 0);
>
> Redundant parentheses detected!

I think you mean superfluous rather than redundant.

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

Reviewed-by: Jim Mattson <jmattson@...gle.com>