Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

[H. Peter Anvin]

On 02/25/2015 01:20 AM, Ingo Molnar wrote:
> I think the fundamental fragility is that we allow the high
> 32 bits to be nonzero.
>
> So could we just zap the high 32 bits of RAX early in the
> entry code, and then from that point on we could both use
> 32-bit ops and won't have to remember the possibility
> either?

We do that, but people keep "optimizing" the zero extend away.  We have 
had this cause a wide-open security hole twice already.  So the extra 
REX prefix is a cheap cost to avoid this happen again.

	-hpa
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