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.
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