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Date: Thu, 9 Jan 2020 14:13:48 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Sean Christopherson' <sean.j.christopherson@...el.com>, Paolo Bonzini <pbonzini@...hat.com> CC: 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 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. 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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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