On Fri, May 02, 2025 at 12:53:36PM -0700, Sean Christopherson wrote: > > So the FRED NMI code is significantly different from the IDT NMI code > > and I really didn't want to go mixing those. > > > > If we get a nested NMI I don't think it'll behave well. > > Ah, because FRED hardwware doesn't have the crazy NMI unblocking behavior, and > the FRED NMI entry code relies on that.
Just so. > But I don't see why we need to care about NMIs from KVM, while they do bounce > through assembly to create a stack frame, the actual CALL is direct to > asm_exc_nmi_kvm_vmx(). If it's just the unwind hint that's needed, that That part is all fine. > arch/x86/entry/entry_64_fred.S | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S > index 29c5c32c16c3..7aff2f0a285f 100644 > --- a/arch/x86/entry/entry_64_fred.S > +++ 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 :-).
