Re: [PATCH] arm64: add unlikely hint to MTE async fault check in el0_svc_common

[Li Qiang]

> Does this result in a measurable performance difference?
> Will

Hi,

I did not observe a measurable performance difference in micro-benchmarks 
(e.g. syscall latency tests), since asynchronous MTE faults are extremely 
rare and the branch is almost never taken.

However, `el0_svc_common()` is a very hot path. Without the `unlikely()` 
hint, the compiler may not optimize for the common case, and after an 
I-cache eviction or branch predictor state reset the fault check can 
be mispredicted. Marking it as `unlikely()` makes the fast path layout 
clearer and reduces the chance of branch misprediction in cold-start 
situations, consistent with how other rare-condition checks are annotated.

Thanks,
Li Qiang