linux-kernel - Re: [PATCH 14/43] KVM: x86: Don't force set BSP bit when local APIC is managed by userspace

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Message-ID: <CAAeT=FzQBOy=ysxXM24_w0O+p5vrAOWCTMxxv__aF2TG=U_AHA@mail.gmail.com>
Date:   Tue, 25 May 2021 23:55:18 -0700
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/43] KVM: x86: Don't force set BSP bit when local APIC
 is managed by userspace

On Fri, Apr 23, 2021 at 5:50 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Don't set the BSP bit in vcpu->arch.apic_base when the local APIC is
> managed by userspace.  Forcing all vCPUs to be BSPs is non-sensical, and
> was dead code when it was added by commit 97222cc83163 ("KVM: Emulate
> local APIC in kernel").  At the time, kvm_lapic_set_base() was invoked
> if and only if the local APIC was in-kernel (and it couldn't be called
> before the vCPU created its APIC).
>
> kvm_lapic_set_base() eventually gained generic usage, but the latent bug
> escaped notice because the only true consumer would be the guest itself
> in the form of an explicit RDMSRs on APs.  Out of Linux, SeaBIOS, and
> EDK2/OVMF, only OVMF consume the BSP bit from the APIC_BASE MSR.  For
> the vast majority of usage in OVMF, BSP confusion would be benign.
> OVMF's BSP election upon SMI rendezvous might be broken, but practically
> no one runs KVM with an out-of-kernel local APIC, let alone does so while
> utilizing SMIs with OVMF.
>
> Fixes: 97222cc83163 ("KVM: Emulate local APIC in kernel")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Reiji Watanabe <reijiw@...gle.com>