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

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Message-ID: <87d0bus1b7.fsf@vitty.brq.redhat.com>
Date:   Wed, 08 Jan 2020 11:13:48 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup

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!

>  }

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

-- 
Vitaly