Re: [PATCH v3 2/4] prtcl: introduce PR_MTE_STORE_ONLY

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, May 02, 2025 at 11:03:02AM -0700, Peter Collingbourne wrote:
> On Fri, May 2, 2025 at 10:37 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > On Thu, Apr 24, 2025 at 10:34:57PM +0200, David Hildenbrand wrote:
> > > On 10.04.25 10:07, Yeoreum Yun wrote:
> > > > PR_MTE_STORE_ONLY is used to restrict the MTE tag check for store
> > > > opeartion only.
> > > >
> > > > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > > > ---
> > > >   include/uapi/linux/prctl.h | 2 ++
> > > >   1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> > > > index 15c18ef4eb11..83ac566251d8 100644
> > > > --- a/include/uapi/linux/prctl.h
> > > > +++ b/include/uapi/linux/prctl.h
> > > > @@ -244,6 +244,8 @@ struct prctl_mm_map {
> > > >   # define PR_MTE_TAG_MASK          (0xffffUL << PR_MTE_TAG_SHIFT)
> > > >   /* Unused; kept only for source compatibility */
> > > >   # define PR_MTE_TCF_SHIFT         1
> > > > +/* MTE tag check store only */
> > > > +# define PR_MTE_STORE_ONLY         (1UL << 19)
> > >
> > > That is the next available bit after PR_MTE_TAG_MASK, correct?
> > >
> > > Would we want to leave some space to grow PR_MTE_TAG_MASK in the future
> > > (could that happen?)?
> >
> > The current mask covers 16 tags (bits 59:56 of a pointer) and given the
> > reluctance to have a tag storage of 4 bits per 16 bytes (3% of RAM), I
> > doubt we'd ever grow this.
> >
> > However, you have a good point, we could indeed leave 32 bits for the
> > tag mask, just in case MTE gets so much traction that someone wants 8
> > bits per tag (and likely a bigger granule than 16 bytes). It doesn't
> > cost us anything to add additional bits from (PR_MTE_TAG_SHIFT + 32).
> 
> If it's 8 bits per tag wouldn't the exclusion mask need to be 256
> bits? I probably wouldn't try to anticipate this case since it would
> likely require a different API anyway.

Yep, not sure what I was thinking. So all good with the original patch.

-- 
Catalin