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

[Jason Baron <jbaron@xxxxxxxxxx>]

On Tue, Oct 04, 2011 at 09:30:01AM -0700, H. Peter Anvin wrote:
> 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
> 

We're currently patching the code at run-time (boot and module load
time), with the 'ideal' no-op anyway, so the initial no-op doesn't
really matter much (other than to save patching if the initial and ideal
match).

-Jason

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