Re: [PATCHv4 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

[Alexander Potapenko <glider@google.com>]

On Wed, Jun 22, 2022 at 6:22 PM Kirill A. Shutemov
<kirill.shutemov@linux.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.
>
Am I right that the desired way to detect the presence of LAM without
enabling it is to check that arch_prctl(ARCH_GET_UNTAG_MASK, ...)
returns zero?

Overall, I think these new arch_prctls should be documented following
the spirit of PR_SET_TAGGED_ADDR_CTRL/PR_GET_TAGGED_ADDR_CTRL
somewhere.

> +
> +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;

One would expect that `arch_prctl(ARCH_ENABLE_TAGGED_ADDR, 0)`
disables tagging for the current process.
Shouldn't this workflow be supported as well?