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Subject: Re: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled at runtime
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.20.1711252002550.2316@nanos>
Date:   Sat, 25 Nov 2017 12:53:55 -0700
Cc:     Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
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Message-Id: <47186975-1DCB-4767-8C4C-0816931E3595@amacapital.net>
References: <20171124172411.19476-1-mingo@kernel.org> <20171124172411.19476-43-mingo@kernel.org> <alpine.DEB.2.20.1711252002550.2316@nanos>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 25, 2017, at 12:18 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Fri, 24 Nov 2017, Ingo Molnar wrote:
>>=20
>> From: Dave Hansen <dave.hansen@linux.intel.com>
>>=20
>> The KAISER CR3 switches are expensive for many reasons.  Not all systems
>> benefit from the protection provided by KAISER.  Some of them can not
>> pay the high performance cost.
>>=20
>> This patch adds a debugfs file.  To disable KAISER, you do:
>>=20
>>    echo 0 > /sys/kernel/debug/x86/kaiser-enabled
>>=20
>> and to re-enable it, you can:
>>=20
>>    echo 1 > /sys/kernel/debug/x86/kaiser-enabled
>>=20
>> This is a *minimal* implementation.  There are certainly plenty of
>> optimizations that can be done on top of this by using ALTERNATIVES
>> among other things.
>=20
> It's not only minimal. It's naive and broken. That thing explodes when
> toggled in the wrong moment. I did not even attempt to debug that, because=

> I think the approach is wrong.
>=20
> If you really want to make it runtime switchable, then:
>=20
> - the shadow tables need to be updated unconditionally. I did not check
>   whether thats done right now, but explosions are simpler to achieve when=

>   switching it back on. Though switching it off crashes as well.
>=20
> - you need to make sure that no task is in user space or on the way to it.=

>   The much I hate stop_machine(), that's probably the right tool.
>   Once everything is in stomp_machine() the switch can be flipped.
>=20
> - the poisoning/unpoisoning of the kernel tables does not need to be done
>   from stop_machine(). That can be done from regular context with a TIF
>   flag, so you can make sure that every task is up to date before
>   returning to user space. Though that needs a lot of thought.
>=20
> For now I really want to see that removed entirely and replaced by a simpl=
e
> boot time switch. We can use the global variable for now and optimize it
> later on.
>=20

Nah, let's do it right: use either X86_FEATURE_WHATEVER or a static_branch. =
 We have nice asm support for both.

Keep in mind that, for a static_branch, actually setting the thing needs to b=
e deferred, but that's straightforward.=

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