[monocasa]

Unpatched systems can leak SIMD/FP state between privilege levels. Pretty fucking high severity since that's where we stick private keys these days.

The cost is more expensive context switches currently since we'll have to fully unload and reload all SIMD/FP state. I'm sure Intel will fix this one in a couple gens.

[hansendc]

The processor has XSAVE (the mechanism that we use to save/restore FPU state and more these days) optimizations internal to the processor that keep it from having to fully reload the FPU state. OSes like Linux have not been doing lazy FPU switching on processors with these optimizations for a long time.

See information about XSAVEOPT and the "Init and Modified Optimizations" in the SDM: intel.com/sdm .

As @luto said above, recent versions of Linux ripped out the lazy handling entirely.

[stryk]

Unpatched meaning systems without the Spectre/Meltdown mitigations enabled? Or is this something unrelated to the previous bugs?

[monocasa]

This is unrelated and requires new patches. Somewhere else in the thread here, someone is saying that Linux isn't vulnerable, but I don't know for sure.

[stryk]

Thanks for clearing that up for me. Wooo boy, another one.