Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

[Joerg Roedel]

Hey Andy,

On Fri, Jan 19, 2018 at 08:30:33AM -0800, Andy Lutomirski wrote:
> I meant that we could have sp0 have a genuinely constant value per
> cpu.  That means that the entry trampoline ends up with RIP, etc in a
> different place depending on whether VM was in use, but the entry
> trampoline code should be able to handle that.  sp1 would have a value
> that varies by task, but it could just point to the top of the stack
> instead of being changed depending on whether VM is in use.  Instead,
> the entry trampoline would offset the registers as needed to keep
> pt_regs in the right place.
> 
> I think you already figured all of that out, though :)

Yes, and after looking a while into it, it would make a nice cleanup for
the entry code. On the other side, it would change the layout for the
in-kernel 'struct pt_regs', so that the user-visible pt_regs ends up
with a different layout than the one we use in the the kernel.

This can certainly be all worked out, but it makes this nice entry-code
cleanup not so nice and clean anymore. At least the work required to
make it work without breaking user-space is not in the scope of this
patch-set.


Regards,

	Joerg