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While performing this investigation, we discovered a recent regression, which resulted in bad performance when booting aarch64 android, making any execution with -smp > 1 slower than -smp 1. In more, an overhead was present when booting x64 android.
In short, QEMU must be built with -mcx16 to ensure we use cmpxchg16 on x64 hosts. Else, any atomic instruction will be serialized, blocking all other vcpus.
Commit introducing the regression: https://gitlab.com/qemu-project/qemu/-/commit/c2bf2ccb266dc9ae4a6da75b845f54535417e109
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