arm64/mte: Add userspace interface for enabling asymmetric mode

arm64/mte: Add userspace interface for enabling asymmetric mode

The architecture provides an asymmetric mode for MTE where tag mismatches
are checked asynchronously for stores but synchronously for loads. Allow
userspace processes to select this and make it available as a default mode
via the existing per-CPU sysfs interface.

Since there PR_MTE_TCF_ values are a bitmask (allowing the kernel to choose
between the multiple modes) and there are no free bits adjacent to the
existing PR_MTE_TCF_ bits the set of bits used to specify the mode becomes
disjoint. Programs using the new interface should be aware of this and
programs that do not use it will not see any change in behaviour.

When userspace requests two possible modes but the system default for the
CPU is the third mode (eg, default is synchronous but userspace requests
either asynchronous or asymmetric) the preference order is:

   ASYMM > ASYNC > SYNC

This situation is not currently possible since there are only two modes and
it is mandatory to have a system default so there could be no ambiguity and
there is no ABI change. The chosen order is basically arbitrary as we do not
have a clear metric for what is better here.

If userspace requests specifically asymmetric mode via the prctl() and the
system does not support it then we will return an error, this mirrors
how we handle the case where userspace enables MTE on a system that does
not support MTE at all and the behaviour that will be seen if running on
an older kernel that does not support userspace use of asymmetric mode.

Attempts to set asymmetric mode as the default mode will result in an error
if the system does not support it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>
Tested-by: Branislav Rankov <branislav.rankov@arm.com>
Link: https://lore.kernel.org/r/20220216173224.2342152-5-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>