Re: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled at runtime

[Andy Lutomirski]

> On Nov 25, 2017, at 12:18 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
>> On Fri, 24 Nov 2017, Ingo Molnar wrote:
>> 
>> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> 
>> 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.

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