Messages in this thread | | | Date | Sat, 25 Nov 2017 21:05:20 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled at runtime |
| |
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.
Thanks,
tglx
| |