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

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Message-ID: <87ee0zvszt.ffs@tglx>
Date:   Thu, 12 May 2022 14:24:22 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        "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 09:02, Peter Zijlstra wrote:
> 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)
>> +{
>
> 	return (lam & 0x3) << X86_CR3_LAM_U57;

This "works" because the hardware ignores LAM_U48 if LAM_U57 is set, but
I'd rather make that exclusive in the prctl() as setting both does not
make any sense.

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

Isn't that true for most new hardware features?

Thanks,

        tglx