linux-kernel - Re: [PATCHv6 06/11] x86/mm: Provide arch

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Date:   Mon, 22 Aug 2022 11:32:48 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Taras Madan <tarasmadan@...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 Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv6 06/11] x86/mm: Provide arch_prctl() interface for LAM

On Mon, Aug 15, 2022 at 6:15 AM Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> Add a couple of arch_prctl() handles:
>
>  - ARCH_ENABLE_TAGGED_ADDR enabled LAM. The argument is required number
>    of tag bits. It is rounded up to the nearest LAM mode that can
>    provide it. For now only LAM_U57 is supported, with 6 tag bits.
>
>  - ARCH_GET_UNTAG_MASK returns untag mask. It can indicates where tag
>    bits located in the address.
>
>  - ARCH_GET_MAX_TAG_BITS returns the maximum tag bits user can request.
>    Zero if LAM is not supported.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>

(with a nit, see below)

> +static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
> +{
> +       int ret = 0;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_LAM))
> +               return -ENODEV;
> +
> +       mutex_lock(&mm->context.lock);
> +
> +       /* Already enabled? */
> +       if (mm->context.lam_cr3_mask) {
> +               ret = -EBUSY;
> +               goto out;
> +       }
> +
> +       if (!nr_bits) {
> +               ret = -EINVAL;
> +               goto out;
> +       } else if (nr_bits <= 6) {

Can you please make this 6 a #define?


> +                       return put_user(6, (unsigned long __user *)arg2);
... and use it at least here (could also express masks in terms of
this number, but maybe it's enough to just declare them in the same
header next to each other).