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? > > > > 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. As discussed offline, we don't have a use-case for this yet, so I don't insist. > -- > Kirill A. Shutemov -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Paul Manicle, Liana Sebastian Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
