linux-kernel - Re: [RFC PATCH v3 28/59] KVM: x86: Check for pending APICv interrupt in kvm_vcpu_has

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Message-ID: <YaUn6pqZHrw4Z8zn@google.com>
Date:   Mon, 29 Nov 2021 19:20:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     isaku.yamahata@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC PATCH v3 28/59] KVM: x86: Check for pending APICv interrupt
 in kvm_vcpu_has_events()

On Thu, Nov 25, 2021, Paolo Bonzini wrote:
> On 11/25/21 01:20, isaku.yamahata@...el.com wrote:
> > From: Sean Christopherson<sean.j.christopherson@...el.com>
> > 
> > Return true for kvm_vcpu_has_events() if the vCPU has a pending APICv
> > interrupt to support TDX's usage of APICv.  Unlike VMX, TDX doesn't have
> > access to vmcs.GUEST_INTR_STATUS and so can't emulate posted interrupts,
> > i.e. needs to generate a posted interrupt and more importantly can't
> > manually move requested interrupts into the vIRR (which it also doesn't
> > have access to).
> 
> Does this mean it is impossible to disable APICv on TDX?  If so, please add
> a WARN.

Yes, APICv is forced.

Rereading this patch, checking only for a pending posted interrupt isn't correct,
a pending interrupt that's below the PPR shouldn't be considered a wake event.

A much better approach would be to have vt_sync_pir_to_irr() redirect to a TDX
implementation to read the PIR but not update the vIRR, that way common x86 doesn't
need to be touched.  Hopefully that can be done in a race-free way.