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

[Thomas Gleixner]

On Sat, 25 Nov 2017, Andy Lutomirski wrote:
> > 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.

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.

Thanks,

	tglx