Re: lkm_map

[Frank van der Linden <fvdl%netbsd.org@localhost>]

Wolfgang Solfrank wrote:
> Hi,
>
> when playing with the new module framework, I tried to load/unload
> the example module.  Loading works like a charm, but when unloading
> the module, the system panics with an assertion failure in
> uvm_km_pgremove_intrsafe at line 473, i.e. the
>
>     KASSERT(end <= VM_MAX_KERNEL_ADDRESS);
>
> I tracked it down to the fact that VM_MAX_KERNEL_ADDRESS is
> defined (in sys/arch/amd64/include/vmparam.h) as 0xffffff8000000000.
> At the same time the lkm_map (which is used to map modules) is
> allocated in sys/arch/amd64/amd64/machdep.c just beyond the kernel
> itself (probably in order to be able to use short(er?) calls from
> the module into the kernel).  The kernel virtual space starts at
> KERNBASE, defined (in sys/arch/amd64/include/param.h) as
> 0xffffffff80000000.  This results in the lkm_map not being part
> of the kernel_map, which then leads to the assertion failure
> observed.
>
> So the question is, why is VM_MAX_KERNEL_ADDRESS defined this low,
> so that the "kernel virtual space", i.e. KERNBASE, is not within
> (VM_MIN_KERNEL_ADDRESS, KERN_MAX_KERNEL_ADDRESS(?
The kernel virtual space is determined by the -cmodel=kernel compile 
flags, that put it in the 4T-2G region. lkm_map is indeed located where 
it is to be able to link modules compiled with the same flags (the issue 
is %rip-relative addressing). Therefore, KERNBASE is 0xffffffff80000000 
(4T-2G).

The total address space that is available to pick kernel data space from 
is 0xffffff8000000000 - KERNBASE. However, that's a very large range, 
and some kernel data structures blew up when I configured the kernel for 
the whole range. Also, I ran into some assumptions that only the lower 
end of the kernel space can be pre-allocated by e.g. kernel text, not 
the upper part, which would need to be the case for amd64.

So I  just used a more limited range at the lower end of the available 
space.

I don't know if the data structure limitations and code assumptions are 
still there, perhaps they aren't anymore.

- Frank