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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@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: [RFCv2 04/10] x86/mm: Introduce X86_THREAD_LAM_U48 and
 X86_THREAD_LAM_U57
In-Reply-To: <20220511070211.GS76023@worktop.programming.kicks-ass.net>
References: <20220511022751.65540-1-kirill.shutemov@linux.intel.com>
 <20220511022751.65540-6-kirill.shutemov@linux.intel.com>
 <20220511070211.GS76023@worktop.programming.kicks-ass.net>
Date:   Thu, 12 May 2022 14:24:22 +0200
Message-ID: <87ee0zvszt.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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