linux-kernel - Re: [RFCv2 04/10] x86/mm: Introduce X86_THREAD_LAM_U48 and X86_THREAD_LAM

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Message-ID: <20220511070211.GS76023@worktop.programming.kicks-ass.net>
Date:   Wed, 11 May 2022 09:02:11 +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 04/10] x86/mm: Introduce X86_THREAD_LAM_U48 and
 X86_THREAD_LAM_U57

On Wed, May 11, 2022 at 05:27:45AM +0300, Kirill A. Shutemov wrote:

> +#define LAM_NONE	0
> +#define LAM_U57		1
> +#define LAM_U48		2

> +#define X86_THREAD_LAM_U48		0x1
> +#define X86_THREAD_LAM_U57		0x2

Seriously pick an order and stick with it. I would suggest keeping the
hardware order and then you can do:

> +static inline unsigned long lam_to_cr3(u8 lam)
> +{
> +	switch (lam) {
> +	case LAM_NONE:
> +		return 0;
> +	case LAM_U57:
> +		return X86_CR3_LAM_U57;
> +	case LAM_U48:
> +		return X86_CR3_LAM_U48;
> +	default:
> +		WARN_ON_ONCE(1);
> +		return 0;
> +	}

	return (lam & 0x3) << X86_CR3_LAM_U57;

> +}
> +
> +static inline u8 cr3_to_lam(unsigned long cr3)
> +{
> +	if (cr3 & X86_CR3_LAM_U57)
> +		return LAM_U57;
> +	if (cr3 & X86_CR3_LAM_U48)
> +		return LAM_U48;
> +	return 0;


	return (cr3 >> X86_CR3_LAM_U57) & 0x3;

> +}

and call it a day, or something.

I'm still not liking LAM(e), I'm thikning it's going to create more
problems than it solves.