linux-kernel - Re: [PATCH v6 00/17] Introducing AMD x2AVIC and hybrid-AVIC modes

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Date:   Tue, 28 Jun 2022 20:20:20 +0700
From:   "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, mlevitsk@...hat.com, seanjc@...gle.com,
        joro@...tes.org, jon.grimm@....com, wei.huang2@....com,
        terry.bowman@....com
Subject: Re: [PATCH v6 00/17] Introducing AMD x2AVIC and hybrid-AVIC modes

Maxim,

On 5/19/2022 5:26 PM, Suravee Suthikulpanit wrote:
> Introducing support for AMD x2APIC virtualization. This feature is
> indicated by the CPUID Fn8000_000A EDX[14], and it can be activated
> by setting bit 31 (enable AVIC) and bit 30 (x2APIC mode) of VMCB
> offset 60h.
> 
> With x2AVIC support, the guest local APIC can be fully virtualized in
> both xAPIC and x2APIC modes, and the mode can be changed during runtime.
> For example, when AVIC is enabled, the hypervisor set VMCB bit 31
> to activate AVIC for each vCPU. Then, it keeps track of each vCPU's
> APIC mode, and updates VMCB bit 30 to enable/disable x2APIC
> virtualization mode accordingly.
> 
> Besides setting bit VMCB bit 30 and 31, for x2AVIC, kvm_amd driver needs
> to disable interception for the x2APIC MSR range to allow AVIC hardware
> to virtualize register accesses.
> 
> This series also introduce a partial APIC virtualization (hybrid-AVIC)
> mode, where APIC register accesses are trapped (i.e. not virtualized
> by hardware), but leverage AVIC doorbell for interrupt injection.
> This eliminates need to disable x2APIC in the guest on system without
> x2AVIC support. (Note: suggested by Maxim)
> 
> Testing for v5:
>    * Test partial AVIC mode by launching a VM with x2APIC mode
>    * Tested booting a Linux VM with x2APIC physical and logical modes upto 512 vCPUs.
>    * Test the following nested SVM test use cases:
> 
>               L0     |    L1   |   L2
>         ----------------------------------
>                 AVIC |    APIC |    APIC
>                 AVIC |    APIC |  x2APIC
>          hybrid-AVIC |  x2APIC |    APIC
>          hybrid-AVIC |  x2APIC |  x2APIC
>               x2AVIC |    APIC |    APIC
>               x2AVIC |    APIC |  x2APIC
>               x2AVIC |  x2APIC |    APIC
>               x2AVIC |  x2APIC |  x2APIC

With the commit 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base"),
APICV/AVIC is now inhibit when the guest kernel boots w/ option "nox2apic" or "x2apic_phys"
due to APICV_INHIBIT_REASON_APIC_ID_MODIFIED.

These cases used to work. In theory, we should be able to allow AVIC works in this case.
Is there a way to modify logic in kvm_lapic_xapic_id_updated() to allow these use cases
to work w/ APICv/AVIC?

Best Regards,
Suravee