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

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Message-ID: <20200109172414.GB15001@linux.intel.com>
Date:   Thu, 9 Jan 2020 09:24:14 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     David Laight <David.Laight@...LAB.COM>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup

On Thu, Jan 09, 2020 at 08:36:24AM -0800, Sean Christopherson wrote:
> On Thu, Jan 09, 2020 at 10:26:30AM -0500, Arvind Sankar wrote:
> > On Thu, Jan 09, 2020 at 02:13:48PM +0000, David Laight wrote:
> > > From: Sean Christopherson
> > > > Sent: 08 January 2020 00:19
> > > > 
> > > > 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);
> > > 
> > > Are you sure this isn't deliberate?
> > > The best code almost certainly comes from also removing the '!= 0'.
> 
> The '!= 0' is truly superfluous, removing it doesn't affect code
> generation.

Actually, it's not completely superfluous.  Functionally the code is
identical, but ordered slightly differently for whatever reason.