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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 14 Oct 2021 12:06:42 -0700
Message-ID: <CAKwvOdkC7ydAWs+nB3cxEOrbb7uEjiyBWg1nOOBtKqaCh3zhBg@mail.gmail.com>
Subject: Re: [BUG] [5.15] Compilation error in arch/x86/kvm/mmu/spte.h with clang-14
To:     Sean Christopherson <seanjc@google.com>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Jim Mattson <jmattson@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, torvic9@mailbox.org,
        "vkuznets@redhat.com" <vkuznets@redhat.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "bp@alien8.de" <bp@alien8.de>
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On Thu, Oct 14, 2021 at 11:31 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Oct 14, 2021, Nathan Chancellor wrote:
> > On Mon, Oct 04, 2021 at 10:12:33AM -0700, Jim Mattson wrote:
> > > On Mon, Oct 4, 2021 at 9:13 AM Nick Desaulniers <ndesaulniers@google.com> wrote:
> > > >
> > > > On Mon, Oct 4, 2021 at 2:49 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> > > > >
> > > > > On 04/10/21 11:30, torvic9@mailbox.org wrote:
> > > > > >
> > > > > >> Paolo Bonzini <pbonzini@redhat.com> hat am 04.10.2021 11:26 geschrieben:
> > > > > >>
> > > > > >>
> > > > > >> On 04/10/21 11:08, torvic9@mailbox.org wrote:
> > > > > >>> I encounter the following issue when compiling 5.15-rc4 with clang-14:
> > > > > >>>
> > > > > >>> In file included from arch/x86/kvm/mmu/mmu.c:27:
> > > > > >>> arch/x86/kvm/mmu/spte.h:318:9: error: use of bitwise '|' with boolean operands [-Werror,-Wbitwise-instead-of-logical]
> > > > > >>>           return __is_bad_mt_xwr(rsvd_check, spte) |
> > > > > >>>                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > >>>                                                    ||
> > > > > >>> arch/x86/kvm/mmu/spte.h:318:9: note: cast one or both operands to int to silence this warning
> > > > > >>
> > > > > >> The warning is wrong, as mentioned in the line right above:
> > >
> > > Casting the bool to an int doesn't seem that onerous.
> >
> > Alternatively, could we just change both of the functions to return u64?
> > I understand that they are being used in boolean contexts only but it
> > seems like this would make it clear that a boolean or bitwise operator
> > on them is acceptable.
>
> If we want to fix this, my vote is for casting to an int and updating the comment

At the least, I think bitwise operations should only be performed on
unsigned types.

> in is_rsvd_spte().  I think I'd vote to fix this?  IIRC KVM has had bitwise goofs
> in the past that manifested as real bugs, it would be nice to turn this on.
>
> Or maybe add a macro to handle this?  E.g.

I think Nathan's suggestion was much cleaner.  If explicit casts are
enough to silence the warning, then I think Jim's suggestion is even
better (though unsigned, not signed int).

>
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 7c0b09461349..38aeb4b21925 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -307,6 +307,12 @@ static inline bool __is_bad_mt_xwr(struct rsvd_bits_validate *rsvd_check,
>         return rsvd_check->bad_mt_xwr & BIT_ULL(pte & 0x3f);
>  }
>
> +/*
> + * Macro for intentional bitwise-OR of two booleans, which requires casting at
> + * least one of the results to an int to suppress -Wbitwise-instead-of-logical.
> + */
> +#define BITWISE_BOOLEAN_OR(a, b) (!!((int)(a) | (int)(b)))
> +
>  static __always_inline bool is_rsvd_spte(struct rsvd_bits_validate *rsvd_check,
>                                          u64 spte, int level)
>  {
> @@ -315,8 +321,8 @@ static __always_inline bool is_rsvd_spte(struct rsvd_bits_validate *rsvd_check,
>          * bits and EPT's invalid memtype/XWR checks to avoid an extra Jcc
>          * (this is extremely unlikely to be short-circuited as true).
>          */
> -       return __is_bad_mt_xwr(rsvd_check, spte) |
> -              __is_rsvd_bits_set(rsvd_check, spte, level);
> +       return BITWISE_BOOLEAN_OR(__is_bad_mt_xwr(rsvd_check, spte),
> +                                 __is_rsvd_bits_set(rsvd_check, spte, level));
>  }
>
>  static inline bool spte_can_locklessly_be_made_writable(u64 spte)


-- 
Thanks,
~Nick Desaulniers