linux-kernel - Re: [PATCH 12/43] KVM: x86: Remove defunct BSP "update" in local APIC reset

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Date:   Tue, 25 May 2021 23:54:59 -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 12/43] KVM: x86: Remove defunct BSP "update" in local APIC reset

On Fri, Apr 23, 2021 at 5:49 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Remove a BSP APIC update in kvm_lapic_reset() that is a glorified and
> confusing nop.  When the code was originally added, kvm_vcpu_is_bsp()
> queried kvm->arch.bsp_vcpu, i.e. the intent was to set the BSP bit in the
> BSP vCPU's APIC.  But, stuffing the BSP bit at INIT was wrong since the
> guest can change its BSP(s); this was fixed by commit 58d269d8cccc ("KVM:
> x86: BSP in MSR_IA32_APICBASE is writable").
>
> In other words, kvm_vcpu_is_bsp() is now purely a reflection of
> vcpu->arch.apic_base.MSR_IA32_APICBASE_BSP, thus the update will always
> set the current value and kvm_lapic_set_base() is effectively a nop if
> the new and old values match.  The RESET case, which does need to stuff
> the BSP for the reset vCPU, is handled by vendor code (though this will
> soon be moved to common code).
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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