Re: [PATCH v13 4/7] arm64: mte: Enable TCO in functions that can read beyond buffer limits

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

On 2/22/21 5:58 PM, Catalin Marinas wrote:
> That's because cpu_hotplug_lock is not a spinlock but a semaphore which
> implies sleeping. I don't think avoiding taking this semaphore
> altogether (as in the *_cpuslocked functions) is the correct workaround.
>

Thinking at it a second time I agree, it is not a good idea avoiding to take the
semaphore in this case.

> The mte_enable_kernel_async() function is called on each secondary CPU
> but we don't really need to attempt to toggle the static key on each of
> them as they all have the same configuration. Maybe do something like:
> 
> 	if (!static_branch_unlikely(&mte_async_mode)))
> 		static_branch_enable(&mte_async_mode);
> 
> so that it's only set on the boot CPU.
> 

This should work, maybe with a comment that if we plan to introduce runtime
switching in between async and sync in future we need to revisit our strategy.

> The alternative is to use a per-CPU mask/variable instead of static
> branches but it's probably too expensive for those functions that were
> meant to improve performance.
> 

I would not go for this approach because the reason why we introduced static
branches instead of having a normal variable saving the state was performances.

> We'll still have an issue with dynamically switching the async/sync mode
> at run-time. Luckily kasan doesn't do this now. The problem is that
> until the last CPU have been switched from async to sync, we can't
> toggle the static label. When switching from sync to async, we need
> to do it on the first CPU being switched.
> 

I totally agree on this point. In the case of runtime switching we might need
the rethink completely the strategy and depends a lot on what we want to allow
and what not. For the kernel I imagine we will need to expose something in sysfs
that affects all the cores and then maybe stop_machine() to propagate it to all
the cores. Do you think having some of the cores running in sync mode and some
in async is a viable solution?

Probably it is worth to discuss it further once we cross that bridge.

> So, I think currently we can set the mte_async_mode label to true in
> mte_enable_kernel_async(), with the 'if' check above. For
> mte_enable_kernel_sync(), don't bother with setting the key to false but
> place a WARN_ONCE if the mte_async_mode is true. We can revisit it if
> kasan ever gains this run-time switch mode.

Indeed, this should work for now.

-- 
Regards,
Vincenzo