how to map a vm_object the same vm address in user and kernel space?


I'm trying to share memory between the kernel and userspace so that I can communicate between kernel threads and userspace threads with concurrent lock-free data structures. I can do this if I use different virtual addresses in the kernel and addresses in userspace. However, this requires that I swizzle any pointers stored in the shared memory region.

Is there a way to fix the virtual address as a kernel address but permit userspace threads to dereference it without triggering a segmentation fault?

This is a sketch of the code that I'm trying to use to map the object into userspace addresses.


struct proc *p = td->td_proc;
vm_map_t map = &p->p_vmspace->vm_map;

// setup *addr to bind to the premapped kernel address space
*addr = kernel_addr_for_obj

// circumvent the address range checks *ugly and probably wrong*
map->max_offset = kernel_map->max_offset;

vm_object_reference(obj);
err = vm_map_find(map,obj,0,(vm_offset_t*)addr, size, 0, prot, prot, 0);
err = vm_map_wire(map, (vm_offset_t) *addr, (vm_offset_t) *addr + size, VM_MAP_WIRE_USER);

At this point the virtual addresses are the same and the virtual address points to the same physical address.
pmap_extract(map->pmap, (vm_offset_t)*addr)


However, when the userspace thread attempt to dereference the first page, it causes a segfault.

looking at the U/S bit in the PTE for the first page, the U bit is cleared.
pte = pmap_pte(map->pmap, (vm_offset_t) *addr);

How do I set this cleanly for all the pages used to back the object?

Thanks

John Giacomoni


--

John.Giacomoni@xxxxxxxxxxxx
University of Colorado at Boulder
Department of Computer Science
Engineering Center, ECCR 1B50
430 UCB
Boulder, CO 80303-0430
USA




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