4 ms·
The solution to this particular vulnerability is intuitive to me: snapshot the current privilege level when we enqueue a branch predictor update and carry that
by quotemstr 1y ago
The solution to this particular vulnerability is intuitive to me: snapshot the current privilege level when we enqueue a branch predictor update and carry that snapshot along with the update itself as it flows through the processor's internal buffers. Same problem you might have in software and the same solution, yes?
- wbl 1y agoThat actually doesn't work. The evaluation of the branch condition may be at some point far away from where the privilege update is recognized and executed. There is no current state to update, it's only recognized in retrospect what the state was. And carrying along data is pricey in a CPU: the instruction pointer isn't even available because of this. You could say we only update the predictor at retirement to solve this. But that can get a little dicy also: the retirement queue would have to track this locally and retirement frees up registers, better be sure it's not the one your jump needs to read. Doable but slightly harder than you might think.