aarch64: Memory tagging modes proposal

[Florian Weimer]

* Yury Khrustalev:

> Are we on the same page about this:
>
> 1) There are two things to configure:
>     - memtag scope: heap, stack, globals (I refer to these as 'components')
>     - memtag mode: sync, async, asymm, auto
>
> 2) The 3 components for the scope (heap, stack, globals) can *in theory*
>    be enabled and disabled independently of each other (so whatever
>    configuration we provide should allow for this independent on / off
>    switch for each of the 3 components).
>
> 3) If memtag protection is enabled for at least one of the 3 components,
>    we need to do 2 things:
>     - prctl(PR_SET_TAGGED_ADDR_CTRL, ...)
>     - start using PROT_MTE for mmap / mprotect for the memory that is
>       relevant for the enabled components
>
> 4) When we do that prctl syscall, we use the scope to determine which
>    flags to use for this syscall.

I think we need a statement of requirements *why* this needs to be
configurable at all, and then work out what that means for the
implementation.

A few possible reasons:

* The tag bits confuse existing software.  (There are various subcases.)

* Existing software assumes page-size access granularity and overreads.

* Tag checking needs to be disabled to meet performance goals.

There might be more compatibility aspects.

Thanks,
Florian