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Subject[PATCH 9/9] bpf,x86: Respect X86_FEATURE_RETPOLINE*
Current BPF codegen doesn't respect X86_FEATURE_RETPOLINE* flags and
unconditionally emits a thunk call, this is sub-optimal and doesn't
match the regular, compiler generated, code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/net/bpf_jit_comp.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)

--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2123,14 +2123,18 @@ static int emit_fallback_jump(u8 **pprog
int err = 0;

#ifdef CONFIG_RETPOLINE
- /* Note that this assumes the the compiler uses external
- * thunks for indirect calls. Both clang and GCC use the same
- * naming convention for external thunks.
- */
- err = emit_jump(&prog, __x86_indirect_thunk_rdx, prog);
-#else
- EMIT2(0xFF, 0xE2); /* jmp rdx */
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
+ /* The AMD retpoline can be easily emitted inline. */
+ EMIT3(0x0F, 0xAE, 0xE8); /* lfence */
+ EMIT2(0xFF, 0xE2); /* jmp rdx */
+ } else {
+ /* Call the retpoline thunk */
+ err = emit_jump(&prog, __x86_indirect_thunk_rdx, prog);
+ }
+ } else
#endif
+ EMIT2(0xFF, 0xE2); /* jmp rdx */
*pprog = prog;
return err;
}

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