linux-kernel - Re: [PATCH] [v2] x86: coco: replace static const cc

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Message-ID: <49a6a810-e793-443e-8bb9-a14b2ef6d676@linux.intel.com>
Date: Tue, 11 Mar 2025 06:53:08 -0700
From: Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Arnd Bergmann <arnd@...nel.org>, 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,
 Tom Lendacky <thomas.lendacky@....com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: Arnd Bergmann <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
 Elena Reshetova <elena.reshetova@...el.com>,
 "Jason A. Donenfeld" <Jason@...c4.com>, Yue Haibing <yuehaibing@...wei.com>,
 Juergen Gross <jgross@...e.com>, David Woodhouse <dwmw@...zon.co.uk>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [v2] x86: coco: replace static const cc_mask with a
 function


On 3/10/25 6:10 AM, Arnd Bergmann 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;
>
> Add a cc_get_mask() function mirroring cc_set_mask() for the one
> user of the variable outside of the coco implementation.
>
> Fixes: a0a8d15a798b ("x86/tdx: Preserve shared bit on mprotect()")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> --

Reviewed-by: Kuppuswamy Sathyanarayanan 
<sathyanarayanan.kuppuswamy@...ux.intel.com>

> v2: use an inline helper instead of a __maybe_unused annotaiton.
> ---
>   arch/x86/include/asm/coco.h          | 10 +++++++++-
>   arch/x86/include/asm/pgtable_types.h |  2 +-
>   2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/coco.h b/arch/x86/include/asm/coco.h
> index aa6c8f8ca958..e7225452963f 100644
> --- a/arch/x86/include/asm/coco.h
> +++ b/arch/x86/include/asm/coco.h
> @@ -15,6 +15,11 @@ enum cc_vendor {
>   extern enum cc_vendor cc_vendor;
>   extern u64 cc_mask;
>   
> +static inline u64 cc_get_mask(void)
> +{
> +	return cc_mask;
> +}
> +
>   static inline void cc_set_mask(u64 mask)
>   {
>   	RIP_REL_REF(cc_mask) = mask;
> @@ -25,7 +30,10 @@ u64 cc_mkdec(u64 val);
>   void cc_random_init(void);
>   #else
>   #define cc_vendor (CC_VENDOR_NONE)
> -static const u64 cc_mask = 0;
> +static inline u64 cc_get_mask(void)
> +{
> +	return 0;
> +}
>   
>   static inline u64 cc_mkenc(u64 val)
>   {
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index c90e9c51edb7..74d461cc8e20 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -179,7 +179,7 @@ enum page_cache_mode {
>   };
>   #endif
>   
> -#define _PAGE_CC		(_AT(pteval_t, cc_mask))
> +#define _PAGE_CC		(_AT(pteval_t, cc_get_mask()))
>   #define _PAGE_ENC		(_AT(pteval_t, sme_me_mask))
>   
>   #define _PAGE_CACHE_MASK	(_PAGE_PWT | _PAGE_PCD | _PAGE_PAT)

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer