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

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

[Sean Christopherson]

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);
 }
 
-- 
2.24.1

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

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> 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);

Redundant parentheses detected!

>  }

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly

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

[Jim Mattson]

On Wed, Jan 8, 2020 at 2:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Sean Christopherson <sean.j.christopherson@intel.com> writes:
>
> > 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);
>
> Redundant parentheses detected!

I think you mean superfluous rather than redundant.

> >  }
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Reviewed-by: Jim Mattson <jmattson@google.com>

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

[David Laight]

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)

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

[Arvind Sankar]

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)
> 

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

[Sean Christopherson]

On Thu, Jan 09, 2020 at 10:26:30AM -0500, Arvind Sankar wrote:
> 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'.

The '!= 0' is truly superfluous, removing it doesn't affect code
generation.

> > 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.

Ya, the bitwise-OR was added in commit 25d92081ae2f ("nEPT: Add nEPT
violation/misconfigration support"), and AFAICT it's unintentional.

That being said, I was a bit hasty in stating that a logical-OR allows for
better optimization, sort of.

For FNAME(prefetch_invalid_gpte) and FNAME(walk_addr_generic), which
branch on the result of is_rsvd_bits_set(), the logical-OR is marginally
better.  FNAME(prefetch_invalid_gpte) is what I initially looked at when
saying "yep, that's better!".

But for walk_shadow_page_get_mmio_spte(), because it aggregates the result
in a loop, the bitwise-OR is better in that it eliminates a Jcc.

And all that being said, there are two vastly superior optimizations that
can be made:

  - Reorder the checks in FNAME(prefetch_invalid_gpte) to perform the
    !PRESENT and !ACCESSED checks before checking the reserved bits, as
    they are both more likely to fail and do not require additional memory
    accesses.

  - Rewrite __is_rsvd_bits_set() to make it templated.  The reserved MT
    check is EPT only, i.e. bad_mt_xwr is always 0 for legacy 32/64-bit
    paging.

So, I'll scrap this patch and send a mini series to effect the above
optimizations.

> > 
> > 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)
> > 

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

[Sean Christopherson]

On Thu, Jan 09, 2020 at 08:36:24AM -0800, Sean Christopherson wrote:
> On Thu, Jan 09, 2020 at 10:26:30AM -0500, Arvind Sankar wrote:
> > 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'.
> 
> The '!= 0' is truly superfluous, removing it doesn't affect code
> generation.

Actually, it's not completely superfluous.  Functionally the code is
identical, but ordered slightly differently for whatever reason.

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

[Paolo Bonzini]

On 09/01/20 17:36, Sean Christopherson wrote:
>>> 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.
> Ya, the bitwise-OR was added in commit 25d92081ae2f ("nEPT: Add nEPT
> violation/misconfigration support"), and AFAICT it's unintentional.

It may not be intentional in this case, but it's certainly the kind of
code that I would have fun writing. :)

Paolo

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

[linmiaohe]

Hi：
>
>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);  }
> 
>--
>2.24.1

On the call chain walk_shadow_page_get_mmio_spte --> is_shadow_zero_bits_set --> __is_rsvd_bits_set, the
return value is used as:
	reserved |= is_shadow_zero_bits_set(vcpu->arch.mmu, spte,
		iterator.level);

But this seems ok because val reserved is bool type.

Thanks.