On Mon, Nov 11, 2024 at 09:26:44AM -0800, Sean Christopherson wrote: > KVM: x86: > > On Mon, Nov 11, 2024, Peter Zijlstra wrote: > > Since there is only a single fastop() function, convert the FASTOP > > stuff from CALL_NOSPEC+RET to JMP_NOSPEC+JMP, avoiding the return > > thunks and all that jazz. > > > > Specifically FASTOPs rely on the return thunk to preserve EFLAGS, > > which not all of them can trivially do (call depth tracing suffers > > here). > > Maybe add an example? Mostly as a reminder of how to reproduce the call depth > issues. > > E.g. booting with "retbleed=force,stuff spectre_v2=retpoline,generic" causes > KVM-Unit-Test's "emulator" test to fail due to flags being clobbered. > > > Objtool strenuously complains about this: > > > > - indirect call without a .rodata, fails to determine JUMP_TABLE, > > annotate > > - fastop functions fall through, exception > > - unreachable instruction after fastop_return, save/restore > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > The original patch works, but with the fixup KVM fails emulation of an ADC and > generates: Bah, I'll go chase it down. Thanks!
