Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation

From: Szabolcs Nagy <[email-masked]>
Date: Fri, 15 May 2020 12:14:00 +0100

The 05/15/2020 11:38, Catalin Marinas wrote:
> On Thu, May 14, 2020 at 12:37:22PM +0100, Catalin Marinas wrote:
> > On Wed, May 13, 2020 at 04:48:46PM +0100, Dave P Martin wrote:
> > > > > > On Wed, Apr 29, 2020 at 05:47:05PM +0100, Dave P Martin wrote:
> > > > > > > On Tue, Apr 21, 2020 at 03:26:03PM +0100, Catalin Marinas wrote:
> > > > > > > > +excludes all tags other than 0. A user thread can enable specific tags
> > > > > > > > +in the randomly generated set using the ``prctl(PR_SET_TAGGED_ADDR_CTRL,
> > > > > > > > +flags, 0, 0, 0)`` system call where ``flags`` contains the tags bitmap
> > > > > > > > +in the ``PR_MTE_TAG_MASK`` bit-field.
> > > > > > > > +
> > > > > > > > +**Note**: The hardware uses an exclude mask but the ``prctl()``
> > > > > > > > +interface provides an include mask. An include mask of ``0`` (exclusion
> > > > > > > > +mask ``0xffff``) results in the CPU always generating tag ``0``.
> > > > > > > 
> > > > > > > Is there no way to make this default to 1 rather than having a magic
> > > > > > > meaning for 0?
> > [...]
> > > The only configuration that doesn't make sense is "no tags allowed", so
> > > I'd argue for explicity blocking that, even if the architeture aliases
> > > that encoding to something else.
> > > 
> > > If we prefer 0 as a default value so that init inherits the correct
> > > value from the kernel without any special acrobatics, then we make it an
> > > exclude mask, with the semantics that the hardware is allowed to
> > > generate any of these tags, but does not have to be capable of
> > > generating all of them.
> > 
> > That's more of a question to the libc people and their preference.
> > We have two options with suboptions:
> > 
> > 1. prctl() gets an exclude mask with 0xffff illegal even though the
> >    hardware accepts it:
> >    a) default exclude mask 0, allowing all tags to be generated by IRG
> >    b) default exclude mask of 0xfffe so that only tag 0 is generated
> > 
> > 2. prctl() gets an include mask with 0 illegal:
> >    a) default include mask is 0xffff, allowing all tags to be generated
> >    b) default include mask 0f 0x0001 so that only tag 0 is generated
> > 
> > We currently have (2) with mask 0 but could be changed to (2.b). If we
> > are to follow the hardware description (which makes more sense to me but
> > I don't write the C library), (1.a) is the most appropriate.
> 
> Thinking some more about this, as we are to expose the GCR_EL1.Excl via
> a ptrace interface as a regset, it makes more sense to move back to an
> exclude mask here with default 0. That would be option 1.a above.

i think the libc has to do a prctl call to set
mte up and at that point it will use whatever
arguments necessary, so 1.a should work (just
like the other options).

likely libc will disable 0 for irg and possibly
one or two other fixed colors (which will have
specific use).

the difference i see between 1 vs 2 is forward
compatibility if the architecture changes (e.g.
adding more tag bits) but then likely new prctl
flag will be needed for handling that so it's
probably not an issue.

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