linux-kernel - Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

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Message-ID: <YTD9kIIzAz34Ieeu@google.com>
Date:   Thu, 2 Sep 2021 16:36:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Tao Xu <tao3.xu@...el.com>, pbonzini@...hat.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On Thu, Sep 02, 2021, Sean Christopherson wrote:
> On Tue, Aug 03, 2021, Xiaoyao Li wrote:
> > On 8/2/2021 11:46 PM, Sean Christopherson wrote:
> > > > > > @@ -5642,6 +5653,31 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
> > > > > >    	return 0;
> > > > > >    }
> > > > > > +static int handle_notify(struct kvm_vcpu *vcpu)
> > > > > > +{
> > > > > > +	unsigned long exit_qual = vmx_get_exit_qual(vcpu);
> > > > > > +
> > > > > > +	if (!(exit_qual & NOTIFY_VM_CONTEXT_INVALID)) {
> > > > > 
> > > > > What does CONTEXT_INVALID mean?  The ISE doesn't provide any information whatsoever.
> > > > 
> > > > It means whether the VM context is corrupted and not valid in the VMCS.
> > > 
> > > Well that's a bit terrifying.  Under what conditions can the VM context become
> > > corrupted?  E.g. if the context can be corrupted by an inopportune NOTIFY exit,
> > > then KVM needs to be ultra conservative as a false positive could be fatal to a
> > > guest.
> > > 
> > 
> > Short answer is no case will set the VM_CONTEXT_INVALID bit.
> 
> But something must set it, otherwise it wouldn't exist.  The condition(s) under
> which it can be set matters because it affects how KVM should respond.  E.g. if
> the guest can trigger VM_CONTEXT_INVALID at will, then we should probably treat
> it as a shutdown and reset the VMCS.

Oh, and "shutdown" would be relative to the VMCS, i.e. if L2 triggers a NOTIFY
exit with VM_CONTEXT_INVALID then KVM shouldn't kill the entire VM.  The least
awful option would probably be to synthesize a shutdown VM-Exit to L1.  That
won't communicate to L1 that vmcs12 state is stale/bogus, but I don't see any way
to handle that via an existing VM-Exit reason :-/

> But if VM_CONTEXT_INVALID can occur if and only if there's a hardware/ucode
> issue, then we can do:
> 
> 	if (KVM_BUG_ON(exit_qual & NOTIFY_VM_CONTEXT_INVALID, vcpu->kvm))
> 		return -EIO;
> 
> Either way, to enable this by default we need some form of documentation that
> describes what conditions lead to VM_CONTEXT_INVALID.