Re: KAISER memory layout (Re: [PATCH 06/23] x86, kaiser: introduce user-mapped percpu areas)

[Thomas Gleixner]

On Thu, 2 Nov 2017, Andy Lutomirski wrote:
> I think we're far enough along here that it may be time to nail down
> the memory layout for real.  I propose the following:
> 
> The user tables will contain the following:
> 
>  - The GDT array.
>  - The IDT.
>  - The vsyscall page.  We can make this be _PAGE_USER.

I rather remove it for the kaiser case.

>  - The TSS.
>  - The per-cpu entry stack.  Let's make it one page with guard pages
> on either side.  This can replace rsp_scratch.
>  - cpu_current_top_of_stack.  This could be in the same page as the TSS.
>  - The entry text.
>  - The percpu IST (aka "EXCEPTION") stacks.

Do you really want to put the full exception stacks into that user mapping?
I think we should not do that. There are two options:

  1) Always use the per-cpu entry stack and switch to the proper IST after
     the CR3 fixup

  2) Have separate per-cpu entry stacks for the ISTs and switch to the real
     ones after the CR3 fixup.

> We can either try to move all of the above into the fixmap or we can
> have the user tables be sparse a la Dave's current approach.  If we do
> it the latter way, I think we'll want to add a mechanism to have holes
> in the percpu space to give the entry stack a guard page.
> 
> I would *much* prefer moving everything into the fixmap, but that's a
> wee bit awkward because we can't address per-cpu data in the fixmap
> using %gs, which makes the SYSCALL code awkward.  But we could alias
> the SYSCALL entry text itself per-cpu into the fixmap, which lets us
> use %rip-relative addressing, which is quite nice.
>
> So I guess my preference is to actually try the fixmap approach.  We
> give the TSS the same aliasing treatment we gave the GDT, and I can
> try to make the entry trampoline work through the fixmap and thus not
> need %gs-based addressing until CR3 gets updated.  (This actually
> saves several cycles of latency.)

Makes a lot of sense.

Thanks,

	tglx