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 20:15:06 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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, 2022-05-18 at 15:39 +0000, Sean Christopherson wrote:
> 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.

I can't disagree with you on this, this was just somewhat a hack I wasn't sure
(and not yet 100% sure I will move forward with) so I cut this corner.

Thanks for the review!

Best regards,
	Maxim Levitsky

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