On Sat, May 03, 2025 at 11:28:37AM -0700, Josh Poimboeuf wrote: > On Sat, May 03, 2025 at 11:50:23AM +0200, Peter Zijlstra wrote: > > > +++ b/arch/x86/entry/entry_64_fred.S > > > @@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm) > > > movq %rsp, %rdi /* %rdi -> pt_regs */ > > > call __fred_entry_from_kvm /* Call the C entry point */ > > > POP_REGS > > > - ERETS > > > + > > > + ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED > > > 1: > > > /* > > > * Objtool doesn't understand what ERETS does, this hint tells it that > > > @@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm) > > > * isn't strictly needed, but it's the simplest form. > > > */ > > > UNWIND_HINT_RESTORE > > > - pop %rbp > > > + leave > > > RET > > > > So this, while clever, might be a problem with ORC unwinding. Because > > now the stack is different depending on the alternative, and we can't > > deal with that. > > > > Anyway, I'll go have a poke on Monday (or Tuesday if Monday turns out to > > be a bank holiday :-). > > Can we just adjust the stack in the alternative? > > ALTERNATIVE "add $64 %rsp", __stringify(ERETS), X86_FEATURE_FRED
Yes, that should work. But I wanted to have a poke at objtool, so it will properly complain about the mistake first.
