Re: Dynamic nop selection breaks boot on Geode LX

["H. Peter Anvin" <hpa@zytor.com>]

On 10/04/2010 08:46 AM, Jason Baron wrote:
> 
> move arch_init_ideal_nop5 later
> 
> arch_init_ideal_nop5() was being called from setup_arch() before
> the exception table was setup. Move it later into
> alternative_instructions().
> 
> Fixes a boot hang on OLPC's XO-1 laptop based on Geode LX
> processor.
> 

This code is fundamentally toxic and needs to be scrapped completely --
it is simply broken beyond repair.

We tried exactly this type of dynamic selection before, and it doesn't
work on broken virtualizers; in particular Microsoft VirtualPC can pass
the exception test and yet fail later.

The end result is very simple: you can always use NOPL on 64 bits, you
can never use NOPL on 32 bits.

66 66 66 66 90 will always *work* (as in, it will never fail) but it's
pretty slow on older CPUs which took a hit on handle prefixes -- but it
might still be faster than a jump on those.  Thus, in your code the JMP
case will never be reached anyway.

There isn't, of course, a classic 5-byte sequence, although the sequence:

	2E 8D 75 26 00

... should work (leal %ds:0(,%esi,1),%esi).  However, 66 ... 90 is
likely to work better on modern processors (although I haven't measured it.)

	-hpa