Re: [PATCH 6/7] KVM: VMX: Provide separate subroutines for invoking NMI vs. IRQ handlers

[Sean Christopherson]

On Wed, Dec 14, 2022, Li, Xin3 wrote:
> > +
> > +	/*
> > +	 * "Restore" RSP from RBP, even though IRET has already unwound
> > RSP to
> > +	 * the correct value.  objtool doesn't know the callee will IRET and,
> > +	 * without the explicit restore, thinks the stack is getting walloped.
> > +	 * Using an unwind hint is problematic due to x86-64's dynamic
> > alignment.
> > +	 */
> > +	mov %_ASM_BP, %_ASM_SP
> > +	pop %_ASM_BP
> > +	RET
> 
> For NMI, after this RET instruction, we continue to block NMIs. IRET instead?

No, IRET has already been done by the kernel-provided handler, e.g. asm_exc_nmi_kvm_vmx()
by way of error_return().  That's why the CALL above (that got snipped) is preceded
by the creation of a synthetic NMI/IRQ stack frame: the target returns from the CALL
via IRET, not RET.