On Mon, Aug 16, 2021 at 9:17 PM Samuel Thibault <samuel.thiba...@gnu.org> wrote: > > > I'd rather see a > > > hurd-specific libpciaccess function to get the pager. > > > > That's better, but we'd have to add that function in both hurd_pci.c and > > x86_pci.c. I don't like the idea of adding Hurd specific code to x86_module > > but there's already a lot of it and we could avoid the existence of struct > > pci_user_data, which I like even less. > > Actually I'm thinking that this is just another case of mremap(). The > underlying question is getting the memory object of a given memory > range, like vm_region does. We need to be careful since we don't want > any process to be able to get the memory object of any other process, > but it does make sense, and would be useful for mremap.
IMO the proper way to get mremap () is vm_remap () — basically like vm_map (), but instead of a memory object to map it takes another (task, address) pair. OSF (or just Apple's?) Mach has it [0], although I'd add 'anywhere' and 'unmap_source' flags to that interface. [0] https://developer.apple.com/documentation/kernel/1585336-vm_remap This makes me think of something cooler though: what if there was an API to create a proxy memory object from a (task, address, size) triple? Then you'd be able to map the new proxy, and there wouldn't be a need for a separate vm_remap () — all without getting direct access to the underlying memory object(s). So, something like this: routine vm_create_proxy ( target_task : vm_task_t; address : vm_address_t; size : vm_size_t; out proxy : memory_object_t); Then mremap () could be implemented like this: vm_create_proxy (mach_task_self (), old_address, old_size, &proxy); vm_deallocate (mach_task_self (), old_address, old_size); vm_map (mach_task_self (), new_address, new_size, ..., proxy, ...); mach_port_deallocate (mach_task_self (), proxy); (Hmm, but what about increasing the map size? This cannot be done securely for non-anonimous mappings, can it?) This could also be used for other fun things, like implementing /proc/pid/mem in a coherent way. (It can be implemented in a sad way without this by making a pager that does vm_read () / vm_write () when asked to retrieve / store a page.) What do you think? Sergey
