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Date:   Wed, 16 Dec 2020 14:10:16 +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 13:47 +0100]:
>On Wed, Dec 16, 2020 at 12:55:25PM +0100, Jessica Yu wrote:
>> +++ Peter Zijlstra [16/12/20 10:26 +0100]:
>> [snip]
>> > > 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.
>>
>> Because the jump label code currently does not allow you to update if
>> the entry resides in an init section. By marking the module init
>> section __init you place it in the .init.text section.
>> jump_label_add_module() detects this (by calling within_module_init())
>> and marks the entry by calling jump_entry_set_init(). Then you have
>> the following sequence of calls (roughly):
>>
>> static_branch_enable
>>  static_key_enable
>>    static_key_enable_cpuslocked
>>      jump_label_update
>>        jump_label_can_update
>>          jump_entry_is_init returns true, so bail out
>>
>> Judging from the comment in jump_label_can_update(), this seems to be
>> intentional behavior:
>>
>> static bool jump_label_can_update(struct jump_entry *entry, bool init)
>> {
>>        /*
>>         * Cannot update code that was in an init text area.
>>         */
>>        if (!init && jump_entry_is_init(entry))
>>                return false;
>>
>
>Only because we're having .init=false, incorrectly. See the other email.

Ah yeah, you're right. I also misread the intention of the if
conditional :/ If we're currently running an init function it's fine,
but after that it will be unsafe.

Btw, your patch seems to work for me, using the test module provided
by Dexuan.

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