linux-kernel - RE: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi

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Message-ID: <BN6PR1101MB2161299749E12D484DE9302BA8049@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Tue, 15 Nov 2022 07:50:49 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: RE: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq()
 for NMI/IRQ reinjection

> > > But what about NMIs, afaict this is all horribly broken for NMIs.
> > >
> > > So the whole VMX thing latches the NMI (which stops NMI recursion),
> right?
> > >
> > > But then you drop out of noinstr code, which means any random
> > > exception can happen (kprobes #BP, hw_breakpoint #DB, or even #PF
> > > due to random nonsense like *SAN). This exception will do IRET and
> > > clear the NMI latch, all before you get to run any of the NMI code.
> >
> > What you said here implies that we have this problem in the existing code.
> > Because a fake iret stack is created to call the NMI handler in the
> > IDT NMI descriptor, which lastly executes the IRET instruction.
> 
> I can't follow; of course the IDT handler terminates with IRET, it has to no?

With FRED, ERETS/ERETU replace IRET, and use bit 28 of the popped CS field
to control whether to unblock NMI. If bit 28 of the field (above the selector)
is 1, ERETS/ERETU unblocks NMIs.

> 
> And yes, the current code appears to suffer the same defect.