linux-kernel - Re: [PATCHv5 06/13] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Date:   Mon, 18 Jul 2022 19:47:44 +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: [PATCHv5 06/13] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

On Wed, Jul 13, 2022 at 1:13 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.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/include/uapi/asm/prctl.h |  3 ++
>  arch/x86/kernel/process_64.c      | 60 ++++++++++++++++++++++++++++++-
>  2 files changed, 62 insertions(+), 1 deletion(-)


> +
> +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;

Hm, I used to think ENODEV is specific to devices, and -EINVAL is more
appropriate here.
On the other hand, e.g. prctl(PR_SET_SPECULATION_CTRL) can also return ENODEV...


>  long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
>  {
>         int ret = 0;
> @@ -829,7 +883,11 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
>         case ARCH_MAP_VDSO_64:
>                 return prctl_map_vdso(&vdso_image_64, arg2);
>  #endif
> -
> +       case ARCH_GET_UNTAG_MASK:
> +               return put_user(task->mm->context.untag_mask,
> +                               (unsigned long __user *)arg2);

Can we have ARCH_GET_UNTAG_MASK return the same error value (ENODEV or
EINVAL) as ARCH_ENABLE_TAGGED_ADDR in the case the host doesn't
support LAM?
After all, the mask does not make much sense in this case.

> +       case ARCH_ENABLE_TAGGED_ADDR:
> +               return prctl_enable_tagged_addr(task->mm, arg2);
>         default:
>                 ret = -EINVAL;
>                 break;
> --
> 2.35.1
>


-- 
Alexander Potapenko
Software Engineer

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