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

Sean Christopherson
2022-12-15T00:27:07Z

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.