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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Thu, 9 Jan 2020 10:26:30 -0500
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Sean Christopherson' <sean.j.christopherson@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup
Message-ID: <20200109152629.GA25610@rani.riverdale.lan>
References: <20200108001859.25254-1-sean.j.christopherson@intel.com>
 <c716f793e22e4885a3dee3c91f93e517@AcuMS.aculab.com>
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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@intel.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'.
> You also don't want to convert the expression result to zero.

The function is static inline bool, so it's almost certainly a mistake
originally. The != 0 is superfluous, but this will get inlined anyway.

> 
> So:
> 	return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) | (rsvd_check->bad_mt_xwr & (1ull << low6));
> The code then doesn't have any branches to get mispredicted.
> 
> 	David
> 
> -
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