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SubjectRe: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled at runtime
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> On Nov 25, 2017, at 1:05 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Sat, 25 Nov 2017, Andy Lutomirski wrote:
>>>> On Nov 25, 2017, at 12:18 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>>>> On Fri, 24 Nov 2017, Ingo Molnar wrote:
>>>>
>>>> From: Dave Hansen <dave.hansen@linux.intel.com>
>>>>
>>>> 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.
>>>>
>>>> This patch adds a debugfs file. To disable KAISER, you do:
>>>>
>>>> echo 0 > /sys/kernel/debug/x86/kaiser-enabled
>>>>
>>>> and to re-enable it, you can:
>>>>
>>>> echo 1 > /sys/kernel/debug/x86/kaiser-enabled
>>>>
>>>> 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.
>>>
>>> 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.
>>>
>>> If you really want to make it runtime switchable, then:
>>>
>>> - 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.
>>>
>>> - 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.
>>>
>>> - 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.
>>>
>>> For now I really want to see that removed entirely and replaced by a simple
>>> boot time switch. We can use the global variable for now and optimize it
>>> later on.
>>>
>>
>> Nah, let's do it right: use either X86_FEATURE_WHATEVER or a
>> static_branch. We have nice asm support for both.
>
> Agreed.
>
>> Keep in mind that, for a static_branch, actually setting the thing needs
>> to be deferred, but that's straightforward.
>
> That's not an issue during boot. That would be an issue for a run time
> switch.

What I mean is: if you modify a static_branch too early, it blows up terribly.
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