linux-kernel - Re: [RFC PATCH v3 02/19] KVM: x86: inhibit APICv/AVIC when the guest and/or host changes apic id/base from the defaults.

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Date:   Wed, 18 May 2022 15:39:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        intel-gfx@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Brijesh Singh <brijesh.singh@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH v3 02/19] KVM: x86: inhibit APICv/AVIC when the guest
 and/or host changes apic id/base from the defaults.

On Wed, May 18, 2022, Maxim Levitsky wrote:
> On Wed, 2022-05-18 at 16:28 +0800, Chao Gao wrote:
> > > struct kvm_arch {
> > > @@ -1258,6 +1260,7 @@ struct kvm_arch {
> > > 	hpa_t	hv_root_tdp;
> > > 	spinlock_t hv_root_tdp_lock;
> > > #endif
> > > +	bool apic_id_changed;
> > 
> > What's the value of this boolean? No one reads it.
> 
> I use it in later patches to kill the guest during nested VM entry 
> if it attempts to use nested AVIC after any vCPU changed APIC ID.

Then the flag should be introduced in the later patch, because (a) it's dead code
if that patch is never merged and (b) it's impossible to review this patch for
correctness without seeing the usage, e.g. setting apic_id_changed isn't guarded
with a lock and so the usage may or may not be susceptible to races.

> > > +	apic->vcpu->kvm->arch.apic_id_changed = true;
> > > +}
> > > +