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[Repost; I forgot to copy linux-kernel earlier. Apologies if you
see it twice.]

Skylake moved to 32 Last Branch Records, from previously 16.
The current call stack LBR implementation reads all LBRs and
also saves/restores them on context switch. This patchkit
adds some optimizations to avoid extra costs in most cases
from the larger number of LBRs for call-stack, unless a nesting
larger than 16 is actually needed. It applies on top of the
earlier Skylake code. Some of the optimization will also benefit
earlier CPUs, such as Haswell.

note: one patch is for perf user space, the rest is kernel.

-Andi



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