[RFC PATCH 1/5] x86: introduce preemption disable prefix

[Nadav Amit <namit@vmware.com>]

It is sometimes beneficial to prevent preemption for very few
instructions, or prevent preemption for some instructions that precede
a branch (this latter case will be introduced in the next patches).

To provide such functionality on x86-64, we use an empty REX-prefix
(opcode 0x40) as an indication that preemption is disabled for the
following instruction.

It is expected that this opcode is not in common use.

Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/entry/entry_64.S            | 10 ++++++++++
 arch/x86/include/asm/nospec-branch.h | 12 ++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index cb8a5893fd33..31d59aad496e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -643,6 +643,16 @@ retint_kernel:
 	jnc	1f
 0:	cmpl	$0, PER_CPU_VAR(__preempt_count)
 	jnz	1f
+
+	/*
+	 * Allow to use hint to prevent preemption on a certain instruction.
+	 * Consider an instruction with the first byte having REX prefix
+	 * without any bits set as an indication for preemption disabled.
+	 */
+	movq    RIP(%rsp), %rax
+	cmpb    $PREEMPT_DISABLE_PREFIX, (%rax)
+	jz	1f
+
 	call	preempt_schedule_irq
 	jmp	0b
 1:
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 80dc14422495..0267611eb247 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -52,6 +52,12 @@
 	jnz	771b;				\
 	add	$(BITS_PER_LONG/8) * nr, sp;
 
+/*
+ * An empty REX-prefix is an indication that preemption should not take place on
+ * this instruction.
+ */
+#define PREEMPT_DISABLE_PREFIX                 (0x40)
+
 #ifdef __ASSEMBLY__
 
 /*
@@ -148,6 +154,12 @@
 #endif
 .endm
 
+.macro preempt_disable_prefix
+#ifdef CONFIG_PREEMPT
+	.byte	PREEMPT_DISABLE_PREFIX
+#endif
+.endm
+
 #else /* __ASSEMBLY__ */
 
 #define ANNOTATE_NOSPEC_ALTERNATIVE				\
-- 
2.17.1