linux-kernel - Re: [PATCH] KVM: x86/mmu: fix comparison of u8 with -1

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Date:   Tue, 26 Nov 2019 09:24:10 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        kvm@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        kernel-janitors@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: fix comparison of u8 with -1

On Tue, Nov 26, 2019 at 8:21 AM Colin King <colin.king@...onical.com> wrote:
>
> From: Colin Ian King <colin.king@...onical.com>
>
> The comparison of the u8 value __entry->u with -1 is always
> going to be false because a __entry-u can never be negative.
> Fix this by casting it to a s8 integer.
>
> Addresses clang warning:
> arch/x86/kvm/./mmutrace.h:360:16: warning: result of comparison
> of constant -1 with expression of type 'u8' (aka 'unsigned char')
> is always false [-Wtautological-constant-out-of-range-compare]

(__entry->u is defined as a u8)

>
> Fixes: 335e192a3fa4 ("KVM: x86: add tracepoints around __direct_map and FNAME(fetch)")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  arch/x86/kvm/mmutrace.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmutrace.h b/arch/x86/kvm/mmutrace.h
> index 7ca8831c7d1a..3466cd528a67 100644
> --- a/arch/x86/kvm/mmutrace.h
> +++ b/arch/x86/kvm/mmutrace.h
> @@ -357,7 +357,7 @@ TRACE_EVENT(
>                   __entry->r ? "r" : "-",
>                   __entry->spte & PT_WRITABLE_MASK ? "w" : "-",
>                   __entry->x ? "x" : "-",
> -                 __entry->u == -1 ? "" : (__entry->u ? "u" : "-"),
> +                 (s8)__entry->u == -1 ? "" : (__entry->u ? "u" : "-"),

Or could compare against 0xFF instead of -1.  Either way, thanks for the patch.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>                   __entry->level, __entry->sptep
>         )
>  );


-- 
Thanks,
~Nick Desaulniers