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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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.

        -hpa


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


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