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

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Message-ID: <BN6PR1101MB216163B12055F02E8C54D2BFA8039@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Sat, 12 Nov 2022 06:35:39 +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

> > > So instead of fixing it, they made it worse :-(
> > >
> > > And now FRED is arguably making it worse again, and people wonder
> > > why I hate virt...
> >
> > Maybe I take it wrong, but FRED doesn't make anything worse. Fred
> > entry code will call external_interrupt() immediately for IRQs.
> 
> But what about NMIs, afaict this is all horribly broken for NMIs.

NMIs are NOT handled by external_interrupt(), which is introduced in patch 4.

The NMI handling is added to kvm_vmx_reinject_nmi_irq() in patch 5 purely for VMX only.

> 
> 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.
> 
> Note how the normal NMI code is very careful to clear DR7 and both kprobes
> and hw_breakpoint know not to accept noinstr code as targets.
> 
> You threw all that out the window.
> 
> Also, NMI is IST, and with FRED it will run on a different stack as well, directly
> calling external_interrupt() doesn't honour that either.
> 
> > You really really don't like the context how VMX dispatches NMI/IRQs
> > (which has been there for a long time), right?
> 
> I really really hate this with a passion. The fact that it's been this way is no
> justification for keeping it. Crap is crap.
> 
> Intel should have taken an example of SVM in this regard, and not doubled
> down and extended this NMI hole to regular IRQs. These are exactly the kind of
> exception delivery trainwrecks FRED is supposed to fix, except in this case it
> appears it doesn't :/