linux-kernel - Re: [PATCH] x86: coco: mark cc_mask as __maybe

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Message-ID: <Z8grEnsAcMrm9sCc@gmail.com>
Date: Wed, 5 Mar 2025 11:44:34 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Arnd Bergmann <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: coco: mark cc_mask as __maybe_unused


* Arnd Bergmann <arnd@...nel.org> wrote:

> From: Arnd Bergmann <arnd@...db.de>
> 
> When extra warnings are enabled, the cc_mask definition in asm/coco.h
> causes a build failure with gcc:
> 
> arch/x86/include/asm/coco.h:28:18: error: 'cc_mask' defined but not used [-Werror=unused-const-variable=]
>    28 | static const u64 cc_mask = 0;
> 
> Mark this one as __maybe_unused.
> 
> Fixes: a0a8d15a798b ("x86/tdx: Preserve shared bit on mprotect()")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/x86/include/asm/coco.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/coco.h b/arch/x86/include/asm/coco.h
> index aa6c8f8ca958..9e9204cfca6f 100644
> --- a/arch/x86/include/asm/coco.h
> +++ b/arch/x86/include/asm/coco.h
> @@ -25,7 +25,7 @@ u64 cc_mkdec(u64 val);
>  void cc_random_init(void);
>  #else
>  #define cc_vendor (CC_VENDOR_NONE)
> -static const u64 cc_mask = 0;
> +static const __maybe_unused u64 cc_mask = 0;

So I detest __maybe_unused with a vengeance: the 'maybe' unnecessarily 
inserts uncertainty & probability language into the text, while there's 
nothing uncertain about this interface or the code. Why cannot the 
compiler figure it out?

Anyway, I'd suggest we change direct usage of cc_mask to a 
get_cc_mask() inline function instead, this will resolve the warning, 
plus it avoids some messy looking variable shadowing in tdx.c AFAICS:

  arch/x86/coco/tdx/tdx.c:static void tdx_setup(u64 *cc_mask)

Thanks,

	Ingo