The 05/27/2020 10:55, Will Deacon wrote:

On Tue, May 26, 2020 at 07:11:53PM -0700, Patrick Daly wrote:

quoted

On Fri, May 22, 2020 at 11:37:15AM +0100, Catalin Marinas wrote:

quoted

The actual question here is what the on/off behaviour is needed for. We
can figure out the best mechanism for this once we know what we want to
achieve. My wild guess above was performance analysis but that can be
toggled by either kernel boot parameter or run-time sysctl (or just the
Kconfig option).

If it is about forcing user space not to use MTE, we may look into some
other sysctl controls (we already have one for the tagged address ABI).

We want to allow the end user to be able to easily "opt out" of MTE in favour
of better power, perf and battery life.

Who is "the end user" in this case?

If MTE is bad enough for power, performance and battery life that we need a
kill switch, then perhaps we shouldn't enable it by default and the few
people that want to use it can build a kernel with it enabled. However, then
I don't really see what MTE buys you over the existing KASAN implementations.

I thought the general idea was that you could run in the (cheap) "async"
mode, and then re-run in the more expensive "sync" mode to further diagnose
any failures. That model seems to work well with these patches, since
reporting is disabled by default. Are you saying that there is a
significant penalty incurred even when reporting is not enabled?

Anyway, we don't offer global runtime/cmdline switches for the vast majority
of other architectural features -- instead, we choose a sensible default,
and I think we should do the same here.

i would not expect mte overhead if userspace processes
don't map anything with PROT_MTE and don't enable tag
checking with prctl. (i.e. userspace runtimes can "opt
out" of mte for better power, perf and battery)

is that not the case?

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