Re: [PATCH 0/6] Introduce CET supervisor state support

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Thu, 2024-07-11 at 15:30 -0700, Dave Hansen wrote:
> > > $BYTES is 24, right?  Did I get anything wrong?
> > 
> > Do we know what the actual memory use is? It would increases the size asked
> > of
> > of the allocator by 24 bytes, but what amount of memory actually gets
> > reserved?
> > 
> > It is sometimes a slab allocated buffer, and sometimes a vmalloc, right? I'm
> > not
> > sure about slab sizes, but for vmalloc if the increase doesn't cross a page
> > size, it will be the same size allocation in reality. Or if it is close to a
> > page size already, it might use a whole extra 4096 bytes.
> 
> Man, I hope I don't have this all mixed up in my head.  Wouldn't be the
> first time.  I _think_ you might be confusing thread_info and
> thread_struct, though.  I know I've gotten them confused before.
> 
> But we get to the 'struct fpu' via:
> 
>         current->thread.fpu
> 
> Where current is a 'task_struct' which is in /proc/slabinfo and 'struct
> thread_struct thread' and 'struct fpu' are embedded in 'task_struct',
> not allocated on their own:

I think thread_struct is always a slab, but the current->thread.fpu.fpstate
pointer can be reallocated to point to a vmalloc in fpstate_realloc(), in the
case of XFD features.

> 
>         task_struct         2958   3018  10048  3 8 ...
> 
> So my current task_struct is 10048 bytes and 3 of them fit in each
> 8-page slab, leaving 2624 bytes to spare.
> 
> I don't think we're too dainty about adding thing to task_struct.  Are we?

So for you there would actually not be any extra memory usage to unconditionally
add 24 bytes to the xstate. But, yes, it all could change for a number of
reasons.

> 
> > So we might be looking at a situation where some tasks get an entire extra
> > page
> > allocated per task, and some get no difference. And only the average is 24
> > bytes
> > increase.
> 
> I think you're right here, at least when it comes to large weirdly-sized
> slabs.  But _so_ many things affect task_struct that I've never seen
> anyone sweat it too much.

Makes sense. Then I can't think of any argument to move from case 2.