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

[hpa]

On December 30, 2018 11:21:07 PM PST, Nadav Amit <namit@xxxxxxxxxx> wrote:
>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)

A 0x40 prefix is *not* a noop. It changes the interpretation of byte registers 4 though 7 from ah, ch, dh, bh to spl, bpl, sil and dil.

It may not matter in your application but:

a. You need to clarify that so is the case, and why;
b. Phrase it differently so others don't propagate the same misunderstanding.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.