Re: x86/clearregs: Register sanitizing at kernel entry for speculation hygiene

[Andi Kleen]

> 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