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Date:   Mon, 5 Sep 2022 17:46:49 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Bharata B Rao <bharata@amd.com>, ananth.narayan@amd.com,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Kostya Serebryany <kcc@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Taras Madan <tarasmadan@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv8 00/11] Linear Address Masking enabling
Message-ID: <YxYZ6Q+JVg79aDNS@hirez.programming.kicks-ass.net>
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On Mon, Sep 05, 2022 at 06:35:17PM +0300, Kirill A. Shutemov wrote:
> What about something like this?
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 803241dfc473..868d2730884b 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -30,8 +30,10 @@ static inline bool pagefault_disabled(void);
>   */
>  #define untagged_addr(mm, addr)	({					\
>  	u64 __addr = (__force u64)(addr);				\
> -	s64 sign = (s64)__addr >> 63;					\
> -	__addr &= (mm)->context.untag_mask | sign;			\
> +	if (static_cpu_has(X86_FEATURE_LAM)) {				\
> +		s64 sign = (s64)__addr >> 63;				\
> +		__addr &= (mm)->context.untag_mask | sign;		\
> +	}								\
>  	(__force __typeof__(addr))__addr;				\
>  })

Well, if you go throught the trouble of adding it, might as well use a
regular static branch and only enable it once there's an actual user,
no?