linux-kernel - Re: [PATCH v2 03/11] KVM: nSVM: explicitly raise KVM_REQ_EVENT on nested VM exit if L1 doesn't intercept interrupts

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Message-ID: <Y9gPT709jG/9SpSq@google.com>
Date:   Mon, 30 Jan 2023 18:41:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     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 03/11] KVM: nSVM: explicitly raise KVM_REQ_EVENT on
 nested VM exit if L1 doesn't intercept interrupts

On Sat, Jan 28, 2023, Sean Christopherson wrote:
> > +	 * If the L2 doesn't intercept interrupts, then
> > +	 * (even if the L2 does use virtual interrupt masking),
> 
> KVM uses "L2" to refer to the thing running at L2.  I think what you are referring
> to here is vmcb12?  And that's controlled by L1.
> 
> > +	 * KVM will use the vmcb02's V_INTR to detect interrupt window.
> 
> s/the vmcb02/vmcb02
> 
> Which of the V_INTR fields does this refer to?  Oooh, you're saying the KVM injects
> a virtual interrupt into L2 using vmcb02 in order to determine when L2 has IRQs
> enabled.
> 
> Why does KVM do that?  Why not pend the actual IRQ directly?

Duh, because KVM needs to gain control in if there are multiple pending events.