linux-kernel - Re: [RFCv2 05/10] x86/mm: Provide untagged

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Date:   Wed, 11 May 2022 09:21:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv2 05/10] x86/mm: Provide untagged_addr() helper

On Wed, May 11, 2022 at 05:27:46AM +0300, Kirill A. Shutemov wrote:
> +#define __untagged_addr(addr, n)	\
> +	((__force __typeof__(addr))sign_extend64((__force u64)(addr), n))
> +
> +#define untagged_addr(addr)	({					\
> +	u64 __addr = (__force u64)(addr);				\
> +	if (__addr >> 63 == 0) {					\
> +		if (current->thread.features & X86_THREAD_LAM_U57)	\
> +			__addr &= __untagged_addr(__addr, 56);		\
> +		else if (current->thread.features & X86_THREAD_LAM_U48)	\
> +			__addr &= __untagged_addr(__addr, 47);		\
> +	}								\
> +	(__force __typeof__(addr))__addr;				\
> +})

Assuming you got your bits in hardware order:

	u64 __addr = addr;
	if ((s64)__addr >= 0) {
		int lam = (current->thread.features >> X86_THREAD_LAM_U57) & 3;
		if (lam)
			__addr &= sign_extend64(__addr, 65 - 9*lam);
	}
	__addr;

has less branches on and should definitely result in better code (or I
need more morning juice).

> +
> +#define untagged_ptr(ptr)	({					\
> +	u64 __ptrval = (__force u64)(ptr);				\
> +	__ptrval = untagged_addr(__ptrval);				\
> +	(__force __typeof__(*(ptr)) *)__ptrval;				\
> +})
>  #endif	/* !__ASSEMBLY__ */
>  
>  #ifdef CONFIG_X86_VSYSCALL_EMULATION
> -- 
> 2.35.1
>