On Tue, 2018-01-09 at 18:28 -0800, Andi Kleen wrote: > From: Andi Kleen <[email protected]> > > [This fixes a boot failure in the earlier patches > so may want to be moved earlier to keep git bisect > happy] > > With the latest tip x86/pti I get oopses when booting > a 64bit VM in qemu with RETPOLINE/gcc7 and PTI enabled. > Something is wrong with the ALTERNATIVE_2 sequence > used in NOSPEC_JMP > > Linus suggested a better sequence that is shorter > and simpler and avoids the problem.
This is just masking a problem which has now been fixed properly elsewhere — by removing the NOPs from the start of the underlying RETPOLINE_JMP sequence, *and* by fixing the alternatives mechanism not to get confused when the altinstr sequence starts with a NOP. I'm not really convinced by the alternative. It's actually *longer*, because the lfence can no longer be tucked away in the space that the full retpoline implementation would have taken. You've prepended a three-byte 'nop' to the full retpoline. And I'm not really sure it's simpler either. We go from "do <this>, or <this> or <this>", with each alternative being a complete and equivalent way to branch to the register, to a more complex matrix based on two conditions. On the whole, given that the actual bug is fixed already, I am inclined to dismiss this — albeit carefully, since it was partly Linus' suggestion — as bikeshedding.
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