arm64: mte: Document that user PSTATE.TCO is ignored by kernel uaccess

On exception entry, the kernel explicitly resets the PSTATE.TCO (tag
check override) so that any kernel memory accesses will be checked (the
bit is restored on exception return). This has the side-effect that the
uaccess routines will not honour the PSTATE.TCO that may have been set
by the user prior to a syscall.

There is no issue in practice since PSTATE.TCO is expected to be used
only for brief periods in specific routines (e.g. garbage collection).
To control the tag checking mode of the uaccess routines, the user will
have to invoke a corresponding prctl() call.

Document the kernel behaviour w.r.t. PSTATE.TCO accordingly.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: df9d7a2 ("arm64: mte: Add Memory Tagging Extension documentation")
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

Author: ctmarinas

Documentation/arm64/memory-tagging-extension.rst

@@ -102,7 +102,9 @@ applications.
 system call) are not checked if the user thread tag checking mode is
 ``PR_MTE_TCF_NONE`` or ``PR_MTE_TCF_ASYNC``. If the tag checking mode is
 ``PR_MTE_TCF_SYNC``, the kernel makes a best effort to check its user
-address accesses, however it cannot always guarantee it.
+address accesses, however it cannot always guarantee it. Kernel accesses
+to user addresses are always performed with an effective ``PSTATE.TCO``
+value of zero, regardless of the user configuration.
 
 Excluding Tags in the ``IRG``, ``ADDG`` and ``SUBG`` instructions
 -----------------------------------------------------------------