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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Tue, Jul 12, 2022 at 03:12:01PM +0200, Alexander Potapenko wrote:
> 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?

Returns -1UL, but yes.

> 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.

The plan is to update man page for the syscall once the interface is
upstream.

> > +
> > +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?

Is there an use-case for it?

I would rather keep the interface minimal. We can always add this in the
future if an use-case comes.

-- 
 Kirill A. Shutemov