Messages in this thread | | | Date | Tue, 9 Jan 2018 17:34:33 -0800 | From | Andi Kleen <> | Subject | Re: x86/clearregs: Register sanitizing at kernel entry for speculation hygiene |
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> I don't like this at all. Once upon a time, Linux syscalls were supposed to be fast. Then we learned about the Meltdown screwup, so we mostly fixed it for real upstream and the distroa seriously half-arsed their own fixes [1]. This came with a big performance cost, but it can be turned off on non-busted hardware. So be it.
That's true, but modern CPUs are also a lot faster/wider than the K8 the fast path was originally designed for. A modern CPU can go through these instructions really fast with a very high IPC because they don't have dependencies or stalls.
So it shouldn't hurt very much.
Also in fact when the fast path was originally written the ABI still had a different caller/callee split which made it more better. Later on it already lost some of its benefits and was less of a win. > But now we're proposing to throw out the whole fast path because it might make it a bit harder to do the most obvious attack. Not very hard, mind you, but a little bit harder. And there's no off switch for less-leaky hardware. No thanks.
Well the off switch is a fast CPU.
-Andi
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