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

[Kirill A. Shutemov]

On Thu, Jul 14, 2022 at 04:28:36PM +0200, Alexander Potapenko wrote:
> On Tue, Jul 12, 2022 at 7:14 PM Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxxxxxxx> 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.
> 
> No, I meant the return value of arch_prctl(), but in fact neither
> seems to be true.
> 
> Right now e.g. for the 5.17 kernel arch_prctl(ARCH_GET_UNTAG_MASK,
> &bits) returns -EINVAL regardless of the underlying hardware.
> A new kernel with your patches will return 0 and set bits=-1UL on both
> non-LAM and LAM-enabled machines. How can we distinguish those?

With CPUID?

-- 
 Kirill A. Shutemov