From: Arvind Sankar <nivedita@alum.mit.edu>
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
Date: Thu, 9 Jan 2020 10:26:30 -0500 [thread overview]
Message-ID: <20200109152629.GA25610@rani.riverdale.lan> (raw)
In-Reply-To: <c716f793e22e4885a3dee3c91f93e517@AcuMS.aculab.com>
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
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
next prev parent reply other threads:[~2020-01-09 15:26 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-08 0:18 [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup Sean Christopherson
2020-01-08 10:13 ` Vitaly Kuznetsov
2020-01-08 17:50 ` Jim Mattson
2020-01-09 14:13 ` David Laight
2020-01-09 15:26 ` Arvind Sankar [this message]
2020-01-09 16:36 ` Sean Christopherson
2020-01-09 17:24 ` Sean Christopherson
2020-01-15 18:20 ` Paolo Bonzini
2020-01-08 8:37 linmiaohe
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