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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Sean Christopherson' <sean.j.christopherson@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
CC:     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
Thread-Topic: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup
Thread-Index: AQHVxbk8+XHUyW1VS0SOYvyU0+7JlafiXjig
Date:   Thu, 9 Jan 2020 14:13:48 +0000
Message-ID: <c716f793e22e4885a3dee3c91f93e517@AcuMS.aculab.com>
References: <20200108001859.25254-1-sean.j.christopherson@intel.com>
In-Reply-To: <20200108001859.25254-1-sean.j.christopherson@intel.com>
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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.

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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