linux-kernel - Re: [PATCH v2 07/11] KVM: x86: add a delayed hardware NMI injection interface

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Date:   Wed, 15 Feb 2023 16:22:15 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Santosh Shukla <santosh.shukla@....com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Sandipan Das <sandipan.das@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>,
        Babu Moger <babu.moger@....com>, linux-kernel@...r.kernel.org,
        Jing Liu <jing2.liu@...el.com>,
        Wyes Karny <wyes.karny@....com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 07/11] KVM: x86: add a delayed hardware NMI injection interface

On Wed, Feb 15, 2023, Sean Christopherson wrote:
> On Tue, Feb 14, 2023, Santosh Shukla wrote:
> > "
> > V_NMI_MASK: Indicates whether virtual NMIs are masked. The processor will set V_NMI_MASK
> > once it takes the virtual NMI. V_NMI_MASK is cleared when the guest successfully completes an
> > IRET instruction or #VMEXIT occurs while delivering the virtual NMI
> > "
> >
> > In my initial implementation I had changed V_NMI_MASK for the SMM scenario [1],
> > This is also not required as HW will save the V_NMI/V_NMI_MASK on 
> > SMM entry and restore them on RSM.
> > 
> > That said the svm_{get,set}_nmi_mask will look something like:

...

> >  static void svm_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked)
> >  {
> >         struct vcpu_svm *svm = to_svm(vcpu);
> > 
> > +       if (is_vnmi_enabled(svm))
> > +               return;
> > +
> >         if (masked) {
> >                 svm->nmi_masked = true;
> >                 svm_set_iret_intercept(svm);
> > 
> > is there any inputs on above approach?
> 
> What happens if software clears the "NMIs are blocked" flag?  If KVM can't clear
> the flag, then we've got problems.  E.g. if KVM emulates IRET or SMI+RSM.  And I
> I believe there are use cases that use KVM to snapshot and reload vCPU state,
> e.g. record+replay?, in which case KVM_SET_VCPU_EVENTS needs to be able to adjust
> NMI blocking too.

Actually, what am I thinking.  Any type of state save/restore will need to stuff
NMI blocking.  E.g. live migration of a VM that is handling an NMI (V_NMI_MASK=1)
_and_ has a pending NMI (V_NMI=1) absolutely needs to set V_NMI_MASK=1 on the dest,
otherwise the pending NMI will get serviced when the guest expects NMIs to be blocked.