RE: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup

[David Laight <David.Laight@ACULAB.COM>]

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)