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

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Date:   Thu, 9 Jan 2020 10:26:30 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Sean Christopherson' <sean.j.christopherson@...el.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 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'.
> 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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