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:

quoted

quoted

+++ 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.