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

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Message-ID: <Y5ppzEEqFB1XqhoJ@google.com>
Date:   Thu, 15 Dec 2022 00:26:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Li, Xin3" <xin3.li@...el.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

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.