x86/alternative: Optimize returns patching
Instead of decoding each instruction in the return sites range only to realize that that return site is a jump to the default return thunk which is needed - X86_FEATURE_RETHUNK is enabled - lift that check before the loop and get rid of that loop overhead. Add comments about what gets patched, while at it. Signed-off-by:Borislav Petkov (AMD) <bp@alien8.de> Acked-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20230512120952.7924-1-bp@alien8.de
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