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

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Message-ID: <BN6PR1101MB2161646D0E7FF2ABCAFD4177A8E19@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Thu, 15 Dec 2022 03:06:39 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [PATCH 6/7] KVM: VMX: Provide separate subroutines for invoking
 NMI vs. IRQ handlers

> > > +	 * "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.