Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

[Yu-cheng Yu]

On Mon, 2019-06-10 at 13:43 -0700, Dave Hansen wrote:
> On 6/10/19 1:27 PM, Yu-cheng Yu wrote:
> > > > If the loader cannot allocate a big bitmap to cover all 5-level
> > > > address space (the bitmap will be large), it can put all legacy lib's
> > > > at lower address.  We cannot do these easily in the kernel.
> > > 
> > > This is actually an argument to do it in the kernel.  The kernel can
> > > always allocate the virtual space however it wants, no matter how large.
> > >  If we hide the bitmap behind a kernel API then we can put it at high
> > > 5-level user addresses because we also don't have to worry about the
> > > high bits confusing userspace.
> > 
> > We actually tried this.  The kernel needs to reserve the bitmap space in the
> > beginning for every CET-enabled app, regardless of actual needs. 
> 
> I don't think this is a problem.  In fact, I think reserving the space
> is actually the only sane behavior.  If you don't reserve it, you
> fundamentally limit where future legacy instructions can go.
> 
> One idea is that we always size the bitmap for the 48-bit addressing
> systems.  Legacy code probably doesn't _need_ to go in the new address
> space, and if we do this we don't have to worry about the gigantic
> 57-bit address space bitmap.
> 
> > On each memory request, the kernel then must consider a percentage of
> > allocated space in its calculation, and on systems with less memory
> > this quickly becomes a problem.
> 
> I'm not sure what you're referring to here?  Are you referring to our
> overcommit limits?

Yes.