linux-kernel - Re: static_branch_enable() does not work from a _

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Date:   Wed, 16 Dec 2020 13:38:05 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dexuan Cui <decui@...rosoft.com>, Ingo Molnar <mingo@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: static_branch_enable() does not work from a __init function?

+++ Peter Zijlstra [16/12/20 10:26 +0100]:
>On Wed, Dec 16, 2020 at 03:54:29AM +0000, Dexuan Cui wrote:
>> PS, I originally found: in arch/x86/kvm/vmx/vmx.c: vmx_init(), it looks
>> like the line "static_branch_enable(&enable_evmcs);" does not take effect
>> in a v5.4-based kernel, but does take effect in the v5.10 kernel in the
>> same x86-64 virtual machine on Hyper-V, so I made the above test module
>> to test static_branch_enable(), and found that static_branch_enable() in
>> the test module does not work with both v5.10 and my v5.4 kernel, if the
>> __init marker is used.

By the way, it probably works now because there was a workaround
merged in v5.10, that mentions this very issue:

commit 064eedf2c50f692088e1418c553084bf9c1432f8
Author: Vitaly Kuznetsov <vkuznets@...hat.com>
Date:   Wed Oct 14 16:33:46 2020 +0200

    KVM: VMX: eVMCS: make evmcs_sanitize_exec_ctrls() work again

    It was noticed that evmcs_sanitize_exec_ctrls() is not being executed
    nowadays despite the code checking 'enable_evmcs' static key looking
    correct. Turns out, static key magic doesn't work in '__init' section
    (and it is unclear when things changed) but setup_vmcs_config() is called
    only once per CPU so we don't really need it to. Switch to checking
    'enlightened_vmcs' instead, it is supposed to be in sync with
    'enable_evmcs'.

    Opportunistically make evmcs_sanitize_exec_ctrls '__init' and drop unneeded
    extra newline from it.

    Reported-by: Yang Weijiang <weijiang.yang@...el.com>
    Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
    Message-Id: <20201014143346.2430936-1-vkuznets@...hat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>