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Date:   Tue, 15 Mar 2022 23:10:34 +0800
From:   Chao Gao <chao.gao@intel.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     Zeng Guang <guang.zeng@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jethro Beekman <jethro@fortanix.com>,
        "Huang, Kai" <kai.huang@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hu, Robert" <robert.hu@intel.com>
Subject: Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID
 unconditionally
Message-ID: <20220315151033.GA6038@gao-cwp>
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On Sun, Mar 13, 2022 at 05:09:08PM +0200, Maxim Levitsky wrote:
>> > > This won't work with nested AVIC - we can't just inhibit a nested guest using its own AVIC,
>> > > because migration happens.
>> > 
>> > I mean because host decided to change its apic id, which it can in theory do any time,
>> > even after the nested guest has started. Seriously, the only reason guest has to change apic id,
>> > is to try to exploit some security hole.
>> 
>> Hi
>> 
>> Thanks for the information.  
>> 
>> IIUC, you mean KVM applies APICv inhibition only to L1 VM, leaving APICv
>> enabled for L2 VM. Shouldn't KVM disable APICv for L2 VM in this case?
>> It looks like a generic issue in dynamically toggling APICv scheme,
>> e.g., qemu can set KVM_GUESTDBG_BLOCKIRQ after nested guest has started.
>> 
>
>That is the problem - you can't disable it for L2, unless you are willing to emulate it in software.
>Or in other words, when nested guest uses a hardware feature, you can't at some point say to it:
>sorry buddy - hardware feature disappeared.

Hi Maxim,

I may miss something. When reading Sean's APICv inhibition cleanups, I
find AVIC is disabled for L1 when nested is enabled (SVM is advertised
to L1). Then, I think the new inhibition introduced for changed xAPIC ID
shouldn't be a problem for L2 VM. Or, you plan to remove
APICV_INHIBIT_REASON_NESTED and expose AVIC to L1?

svm_vcpu_after_set_cpuid:
                /*
                 * Currently, AVIC does not work with nested virtualization.
                 * So, we disable AVIC when cpuid for SVM is set in the L1 guest.
                 */
                if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
                        kvm_request_apicv_update(vcpu->kvm, false,
                                                 APICV_INHIBIT_REASON_NESTED);