Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized,don't nop it out

From: H. Peter Anvin
Date: Tue Oct 04 2011 - 12:31:14 EST

On 10/04/2011 07:10 AM, Jason Baron wrote:
> 1) The jmp +0, is a 'safe' no-op that I know is going to initially
> boot for all x86. I'm not sure if there is a 5-byte nop that works on
> all x86 variants - but by using jmp +0, we make it much easier to debug
> cases where we may be using broken no-ops.

There are *plenty*. jmp+0 is about as pessimal as you can get.

The current recommendation when you don't know the CPU you're running at is:

3E 8D 74 26 00 (GENERIC_NOP5_ATOMIC)

... on 32 bits and ...

0F 1F 44 00 00 (P6_NOP5_ATOMIC)

... on 64 bits.


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at