> On Nov 25, 2017, at 1:05 PM, Thomas Gleixner <[email protected]> wrote: > > On Sat, 25 Nov 2017, Andy Lutomirski wrote: >>>> On Nov 25, 2017, at 12:18 PM, Thomas Gleixner <[email protected]> wrote: >>>> On Fri, 24 Nov 2017, Ingo Molnar wrote: >>>> >>>> From: Dave Hansen <[email protected]> >>>> >>>> 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.
