linux-kernel - Re: [PATCH] kvm: x86: mmu: Add cast to negated bitmasks in update_permission

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Message-ID: <d6a99e26-b100-715b-2615-b77259ec8397@redhat.com>
Date:   Tue, 19 Jun 2018 19:35:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Joe Perches <joe@...ches.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Matthias Kaehlcke <mka@...omium.org>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>, hpa@...or.com,
        x86@...nel.org, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86: mmu: Add cast to negated bitmasks in
 update_permission_bitmask()

On 19/06/2018 19:23, Joe Perches wrote:
> On Tue, 2018-06-19 at 10:08 -0700, Nick Desaulniers wrote:
>> On Tue, Jun 19, 2018 at 8:19 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>>
>>> On 15/06/2018 20:45, Nick Desaulniers wrote:
>>>>>
>>>>>> In any case I think it it preferable to fix the code over disabling
>>>>>> the warning, unless the warning is bogus or there are just too many
>>>>>> occurrences.
>>>>>
>>>>> Maybe.
>>>>
>>>> Spurious warning today, actual bug tomorrow?  I prefer to not to
>>>> disable warnings wholesale.  They don't need to find actual bugs to be
>>>> useful.  Flagging code that can be further specified does not hurt.
>>>> Part of the effort to compile the kernel with different compilers is
>>>> to add warning coverage, not remove it.  That said, there may be
>>>> warnings that are never useful (or at least due to some invariant that
>>>> only affects the kernel).  I cant think of any off the top of my head,
>>>> but I'm also not sure this is one.
>>>
>>> This one really makes the code uglier though, so I'm not really inclined
>>> to applying the patch.
>>
>> Note that of the three variables (w, u, x), only u is used later on.
>> What about declaring them as negated with the cast, that way there's
>> no cast in a ternary?
> 
> It'd be simpler to cast in the BYTE_MASK macro itself

I don't think that would work, as the ~ would be done on a zero-extended
signed int.

Paolo

> Ex:
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index d594690d8b95..53673ad4b295 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -4261,8 +4261,9 @@ static void update_permission_bitmask(struct
>> kvm_vcpu *vcpu,
>>  {
>>         unsigned byte;
>>
>> -       const u8 x = BYTE_MASK(ACC_EXEC_MASK);
>> -       const u8 w = BYTE_MASK(ACC_WRITE_MASK);
>> +       const u8 x_not = (u8)~BYTE_MASK(ACC_EXEC_MASK);
>> +       const u8 w_not = (u8)~BYTE_MASK(ACC_WRITE_MASK);
>> +       const u8 u_not = (u8)~BYTE_MASK(ACC_USER_MASK);
>>         const u8 u = BYTE_MASK(ACC_USER_MASK);
>>
>>
>>         bool cr4_smep = kvm_read_cr4_bits(vcpu, X86_CR4_SMEP) != 0;
>> @@ -4278,11 +4279,11 @@ static void update_permission_bitmask(struct
>> kvm_vcpu *vcpu,
>>                  */
>>
>>                 /* Faults from writes to non-writable pages */
>> -               u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
>> +               u8 wf = (pfec & PFERR_WRITE_MASK) ? w_not : 0;
>>                 /* Faults from user mode accesses to supervisor pages */
>> -               u8 uf = (pfec & PFERR_USER_MASK) ? ~u : 0;
>> +               u8 uf = (pfec & PFERR_USER_MASK) ? u_not : 0;
>>                 /* Faults from fetches of non-executable pages*/
>> -               u8 ff = (pfec & PFERR_FETCH_MASK) ? ~x : 0;
>> +               u8 ff = (pfec & PFERR_FETCH_MASK) ? x_not : 0;
>>                 /* Faults from kernel mode fetches of user pages */
>>                 u8 smepf = 0;
>>                 /* Faults from kernel mode accesses of user pages */
>>
>>
>> Maybe you have a better naming scheme than *_not ? What do you think?
> 
> It'd be nicer to cast in the BYTE_MASK macro
> and using "unsigned byte;" is misleading at best.
> 
> ---
>  arch/x86/kvm/mmu.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index d594690d8b95..201711aa99b9 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4246,15 +4246,14 @@ reset_ept_shadow_zero_bits_mask(struct kvm_vcpu *vcpu,
>  				    boot_cpu_data.x86_phys_bits, execonly);
>  }
>  
> -#define BYTE_MASK(access) \
> -	((1 & (access) ? 2 : 0) | \
> -	 (2 & (access) ? 4 : 0) | \
> -	 (3 & (access) ? 8 : 0) | \
> -	 (4 & (access) ? 16 : 0) | \
> -	 (5 & (access) ? 32 : 0) | \
> -	 (6 & (access) ? 64 : 0) | \
> -	 (7 & (access) ? 128 : 0))
> -
> +#define BYTE_MASK(access)				\
> +	((u8)(((access) & 1 ?   2 : 0) |		\
> +	      ((access) & 2 ?   4 : 0) |		\
> +	      ((access) & 3 ?   8 : 0) |		\
> +	      ((access) & 4 ?  16 : 0) |		\
> +	      ((access) & 5 ?  32 : 0) |		\
> +	      ((access) & 6 ?  64 : 0) |		\
> +	      ((access) & 7 ? 128 : 0)))
>  
>  static void update_permission_bitmask(struct kvm_vcpu *vcpu,
>  				      struct kvm_mmu *mmu, bool ept)
>