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Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <4D260352-A9FF-47F2-B3B2-0A87DF16CB70@vmware.com>
Date:   Mon, 5 Nov 2018 11:03:49 -0800
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
Content-Transfer-Encoding: quoted-printable
Message-Id: <D67F7C19-DF59-4405-A5A3-B56C0E491B41@amacapital.net>
References: <20181102232946.98461-1-namit@vmware.com> <20181102232946.98461-3-namit@vmware.com> <20181105140925.GD22467@hirez.programming.kicks-ass.net> <BB6FC627-C152-4C4C-A6D4-FA7F42F2610D@amacapital.net> <4D260352-A9FF-47F2-B3B2-0A87DF16CB70@vmware.com>
To:     Nadav Amit <namit@vmware.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 5, 2018, at 9:49 AM, Nadav Amit <namit@vmware.com> wrote:
>=20
> From: Andy Lutomirski
> Sent: November 5, 2018 at 5:22:32 PM GMT
>> To: Peter Zijlstra <peterz@infradead.org>
>> Cc: Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@redhat.com>, linux-=
kernel@vger.kernel.org, x86@kernel.org, H. Peter Anvin <hpa@zytor.com>, Thom=
as Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hanse=
n <dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, Kees Coo=
k <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.com>, Masami Hiram=
atsu <mhiramat@kernel.org>
>> Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during e=
arly_init
>>=20
>>=20
>>=20
>>>> On Nov 5, 2018, at 6:09 AM, Peter Zijlstra <peterz@infradead.org> wrote=
:
>>>>=20
>>>> On Fri, Nov 02, 2018 at 04:29:41PM -0700, Nadav Amit wrote:
>>>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.=
c
>>>> index aac0c1f7e354..367c1d0c20a3 100644
>>>> --- a/arch/x86/kernel/jump_label.c
>>>> +++ b/arch/x86/kernel/jump_label.c
>>>> @@ -52,7 +52,13 @@ static void __ref __jump_label_transform(struct jump=
_entry *entry,
>>>>  jmp.offset =3D jump_entry_target(entry) -
>>>>           (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>>>>=20
>>>> -    if (early_boot_irqs_disabled)
>>>> +    /*
>>>> +     * As long as we are in early boot, we can use text_poke_early(), w=
hich
>>>> +     * is more efficient: the memory was still not marked as read-only=
 (it
>>>> +     * is only marked after poking_init()). This also prevents us from=
 using
>>>> +     * text_poke() before poking_init() is called.
>>>> +     */
>>>> +    if (!early_boot_done)
>>>>      poker =3D text_poke_early;
>>>>=20
>>>>  if (type =3D=3D JUMP_LABEL_JMP) {
>>>=20
>>> It took me a while to untangle init/maze^H^Hin.c... but I think this
>>> is all we need:
>>>=20
>>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c=

>>> index aac0c1f7e354..ed5fe274a7d8 100644
>>> --- a/arch/x86/kernel/jump_label.c
>>> +++ b/arch/x86/kernel/jump_label.c
>>> @@ -52,7 +52,12 @@ static void __ref __jump_label_transform(struct jump_=
entry *entry,
>>>  jmp.offset =3D jump_entry_target(entry) -
>>>           (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>>>=20
>>> -    if (early_boot_irqs_disabled)
>>> +    /*
>>> +     * As long as we're UP and not yet marked RO, we can use
>>> +     * text_poke_early; SYSTEM_BOOTING guarantees both, as we switch to=

>>> +     * SYSTEM_SCHEDULING before going either.
>>> +     */
>>> +    if (system_state =3D=3D SYSTEM_BOOTING)
>>>      poker =3D text_poke_early;
>>>=20
>>>  if (type =3D=3D JUMP_LABEL_JMP) {
>>=20
>> Can we move this logic into text_poke() and get rid of text_poke_early()?=

>=20
> This will negatively affect poking of modules doing module loading, e.g.,
> apply_paravirt(). This can be resolved by keeping track when the module is=

> write-protected and giving a module parameter to text_poke(). Does it wort=
h
> the complexity?

Probably not.

OTOH, why does alternative patching need text_poke() at all?  Can=E2=80=99t i=
t just write to the text?

>=20
>> FWIW, alternative patching was, at some point, a significant fraction of
>> total boot time in some cases. This was probably mostly due to unnecessar=
y
>> sync_core() calls. Although I think this was reported on a VM, and
>> sync_core() used to be *extremely* expensive on a VM, but that=E2=80=99s f=
ixed
>> now, and it even got backported, I think.
>>=20
>> (Hmm. Maybe we can also make jump label patching work in early boot, too!=
)
>=20
> It may be possible to resolve the dependencies between poking_init() and t=
he
> other *_init(). I first considered doing that, yet, it makes the code very=

> fragile, and I don=E2=80=99t see the value in getting rid of text_poke_ear=
ly() from
> security or simplicity point of views. Let me know if you think otherwise.=

>=20
> Regards,
> Nadav