[RFC v2 1/6] x86: introduce kernel restartable sequence

[Nadav Amit]

It is sometimes beneficial to have a restartable sequence - very few
instructions which if they are preempted jump to a predefined point.

To provide such functionality on x86-64, we use an empty REX-prefix
(opcode 0x40) as an indication for instruction in such a sequence. Before
calling the schedule IRQ routine, if the "magic" prefix is found, we
call a routine to adjust the instruction pointer.  It is expected that
this opcode is not in common use.

The following patch will make use of this function. Since there are no
other users (yet?), the patch does not bother to create a general
infrastructure and API that others can use for such sequences. Yet, it
should not be hard to make such extension later.

Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 arch/x86/entry/entry_64.S            | 16 ++++++++++++++--
 arch/x86/include/asm/nospec-branch.h | 12 ++++++++++++
 arch/x86/kernel/traps.c              |  7 +++++++
 3 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1f0efdb7b629..e144ff8b914f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -644,12 +644,24 @@ retint_kernel:
 	/* Interrupts are off */
 	/* Check if we need preemption */
 	btl	$9, EFLAGS(%rsp)		/* were interrupts off? */
-	jnc	1f
+	jnc	2f
 0:	cmpl	$0, PER_CPU_VAR(__preempt_count)
+	jnz	2f
+
+	/*
+	 * Allow to use restartable code sections in the kernel. Consider an
+	 * instruction with the first byte having REX prefix without any bits
+	 * set as an indication for an instruction in such a section.
+	 */
+	movq    RIP(%rsp), %rax
+	cmpb    $KERNEL_RESTARTABLE_PREFIX, (%rax)
 	jnz	1f
+	mov	%rsp, %rdi
+	call	restart_kernel_rseq
+1:
 	call	preempt_schedule_irq
 	jmp	0b
-1:
+2:
 #endif
 	/*
 	 * The iretq could re-enable interrupts:
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index dad12b767ba0..be4713ef0940 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -54,6 +54,12 @@
 	jnz	771b;				\
 	add	$(BITS_PER_LONG/8) * nr, sp;
 
+/*
+ * An empty REX-prefix is an indication that this instruction is part of kernel
+ * restartable sequence.
+ */
+#define KERNEL_RESTARTABLE_PREFIX		(0x40)
+
 #ifdef __ASSEMBLY__
 
 /*
@@ -150,6 +156,12 @@
 #endif
 .endm
 
+.macro restartable_seq_prefix
+#ifdef CONFIG_PREEMPT
+	.byte	KERNEL_RESTARTABLE_PREFIX
+#endif
+.endm
+
 #else /* __ASSEMBLY__ */
 
 #define ANNOTATE_NOSPEC_ALTERNATIVE				\
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 85cccadb9a65..b1e855bad5ac 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -59,6 +59,7 @@
 #include <asm/fpu/xstate.h>
 #include <asm/vm86.h>
 #include <asm/umip.h>
+#include <asm/nospec-branch.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
@@ -186,6 +187,12 @@ int fixup_bug(struct pt_regs *regs, int trapnr)
 	return 0;
 }
 
+asmlinkage __visible void restart_kernel_rseq(struct pt_regs *regs)
+{
+	if (user_mode(regs) || *(u8 *)regs->ip != KERNEL_RESTARTABLE_PREFIX)
+		return;
+}
+
 static nokprobe_inline int
 do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,
 		  struct pt_regs *regs,	long error_code)
-- 
2.17.1