[PATCH 6/9] x86/alternative: Try inline spectre_v2=retpoline,amd

[Peter Zijlstra <peterz@infradead.org>]

Try and replace retpoline thunk calls with:

  lfence
  call    *%\reg

for spectre_v2=retpoline,amd.

Specifically, the sequence above is 5 bytes for the low 8 registers,
but 6 bytes for the high 8 registers. This means that unless the
compilers prefix stuff the call with higher registers this replacement
will fail.

Luckily GCC strongly favours RAX for the indirect calls and most (95%+
for defconfig-x86_64) will be converted. OTOH clang strongly favours
R11 and almost nothing gets converted.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/alternative.c |   24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -389,15 +389,13 @@ static int emit_indirect(int op, int reg
  *
  *   CALL *%\reg
  *
+ * It also tries to inline spectre_v2=retpoline,amd when size permits.
  */
 static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
 {
+	u8 cc, op = insn->opcode.bytes[0];
 	void (*target)(void);
 	int reg, ret, i = 0;
-	u8 op, cc;
-
-	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
-		return -1;
 
 	target = addr + insn->length + insn->immediate.value;
 	reg = (target - &__x86_indirect_thunk_rax) /
@@ -406,7 +404,22 @@ static int patch_retpoline(void *addr, s
 	if (WARN_ON_ONCE(reg & ~0xf))
 		return -1;
 
-	op = insn->opcode.bytes[0];
+	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
+		/*
+		 * Can't do nothing about the Jcc case here.
+		 */
+		if (op != JMP32_INSN_OPCODE && op != CALL_INSN_OPCODE)
+			return -1;
+
+		bytes[i++] = 0x0f;
+		bytes[i++] = 0xae;
+		bytes[i++] = 0xe8; /* lfence */
+
+		goto indirect;
+	}
+
+	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
+		return -1;
 
 	/*
 	 * Convert:
@@ -430,6 +443,7 @@ static int patch_retpoline(void *addr, s
 		op = JMP32_INSN_OPCODE;
 	}
 
+indirect:
 	ret = emit_indirect(op, reg, bytes + i);
 	if (ret < 0)
 		return ret;