linux-kernel - Re: [PATCH 10/43] KVM: VMX: Move init_vmcs() invocation to vmx_vcpu

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Date:   Tue, 18 May 2021 22:33:22 -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 10/43] KVM: VMX: Move init_vmcs() invocation to vmx_vcpu_reset()

On Fri, Apr 23, 2021 at 5:48 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Initialize constant VMCS state in vcpu_vcpu_reset() instead of in
> vmx_vcpu_create(), which allows for the removal of the open coded "vCPU
> load" sequence since ->vcpu_reset() is invoked while the vCPU is properly
> loaded (which is the entire point of vCPU reset...).
>
> Deferring initialization is effectively a nop as it's impossible to
> safely access the VMCS between the current call site and its new home, as
> both the vCPU and the pCPU are put immediately after init_vmcs(), i.e.
> the VMCS isn't guaranteed to be loaded.
>
> Note, task preemption is not a problem as vmx_sched_in() _can't_ touch
> the VMCS as ->sched_in() is invoked before the vCPU, and thus VMCS, is
> reloaded.  I.e. the preemption path also can't consume VMCS state.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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