[PATCH v4 10/11] tools/objtool: Check for use of the ENQCMD instruction in the kernel

[Fenghua Yu <fenghua.yu@intel.com>]

The ENQCMD implicitly accesses the PASID_MSR to fill in the pasid field
of the descriptor being submitted to an accelerator. But there is no
precise (and stable across kernel changes) point at which the PASID_MSR
is updated from the value for one task to the next.

Kernel code that uses accelerators must always use the ENQCMDS instruction
which does not access the PASID_MSR.

Check for use of the ENQCMD instruction in the kernel and warn on its
usage.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
v3:
- Add Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>

v2:
- Simplify handling ENQCMD (PeterZ and Josh)

 tools/objtool/arch/x86/decode.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index c10ef78df050..479e769ca324 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -112,7 +112,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 	const struct elf *elf = file->elf;
 	struct insn insn;
 	int x86_64, ret;
-	unsigned char op1, op2,
+	unsigned char op1, op2, op3,
 		      rex = 0, rex_b = 0, rex_r = 0, rex_w = 0, rex_x = 0,
 		      modrm = 0, modrm_mod = 0, modrm_rm = 0, modrm_reg = 0,
 		      sib = 0, /* sib_scale = 0, */ sib_index = 0, sib_base = 0;
@@ -139,6 +139,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
 	op1 = insn.opcode.bytes[0];
 	op2 = insn.opcode.bytes[1];
+	op3 = insn.opcode.bytes[2];
 
 	if (insn.rex_prefix.nbytes) {
 		rex = insn.rex_prefix.bytes[0];
@@ -491,6 +492,14 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 			/* nopl/nopw */
 			*type = INSN_NOP;
 
+		} else if (op2 == 0x38 && op3 == 0xf8) {
+			if (insn.prefixes.nbytes == 1 &&
+			    insn.prefixes.bytes[0] == 0xf2) {
+				/* ENQCMD cannot be used in the kernel. */
+				WARN("ENQCMD instruction at %s:%lx", sec->name,
+				     offset);
+			}
+
 		} else if (op2 == 0xa0 || op2 == 0xa8) {
 
 			/* push fs/gs */
-- 
2.35.1