Re: [RFC KVM 06/27] KVM: x86: Exit KVM isolation on IRQ entry

From: Peter Zijlstra
Date: Tue May 14 2019 - 03:09:20 EST

On Mon, May 13, 2019 at 11:13:34AM -0700, Andy Lutomirski wrote:
> On Mon, May 13, 2019 at 9:28 AM Alexandre Chartre
> <alexandre.chartre@xxxxxxxxxx> wrote:

> > Actually, I am not sure this is effectively useful because the IRQ
> > handler is probably faulting before it tries to exit isolation, so
> > the isolation exit will be done by the kvm page fault handler. I need
> > to check that.
> >
> The whole idea of having #PF exit with a different CR3 than was loaded
> on entry seems questionable to me. I'd be a lot more comfortable with
> the whole idea if a page fault due to accessing the wrong data was an
> OOPS and the code instead just did the right thing directly.

So I've ran into this idea before; it basically allows a lazy approach
to things.

I'm somewhat conflicted on things, on the one hand, changing CR3 from
#PF is a natural extention in that #PF already changes page-tables (for
userspace / vmalloc etc..), on the other hand, there's a thin line
between being lazy and being sloppy.

If we're going down this route; I think we need a very coherent design
and strong rules.