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@xxxxxxxxx>
> > ---
> > 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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