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

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

[Colin King]

From: Colin Ian King <colin.king@canonical.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]

Fixes: 335e192a3fa4 ("KVM: x86: add tracepoints around __direct_map and FNAME(fetch)")
Signed-off-by: Colin Ian King <colin.king@canonical.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" : "-"),
 		  __entry->level, __entry->sptep
 	)
 );
-- 
2.24.0

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

[Nick Desaulniers]

On Tue, Nov 26, 2019 at 8:21 AM Colin King <colin.king@canonical.com> wrote:
>
> From: Colin Ian King <colin.king@canonical.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@canonical.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@google.com>

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


-- 
Thanks,
~Nick Desaulniers

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

[linmiaohe]

> The comparison of the u8 value __entry->u with -1 is always going to be false because
> a __entry-u can never be negative.

s/__entry-u/__entry->u/

Thanks.