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

Li, Xin3
2022-12-15T05:19:09Z

> > > > + * "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.
>
>
> You're right, this assembly makes another call into the asm entry point, which
> returns here with IRET.

Like IRET for IDT, we _need_ ERETS for FRED to unblock NMI ASAP. However
a FRED stack frame has way more information than an IDT stack frame,
thus it's complicated to create a FRED stack frame with assembly.

So I prefer "INT $2" for FRED now.

I will post the FRED patch set soon, lets sync up on this afterwards.

Xin