Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 04 Dec 2003 14:31:19 -0500 | From | Tristan Van Berkom <> | Subject | How do I share large portions of memory with "user land" ? |
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Hello all, This is one of those "How do I ..." questions about writing a kernel module/driver.
My question is already well phrased and andswered in an email archived from a few years ago: http://www.ussg.iu.edu/hypermail/linux/kernel/0005.2/0505.html
Andi > The traditional linux way is to implement mmap for Andi > your character device, vmalloc the memory in kernel Andi > and supply it to the user process via mmap.
That means (I'm not mistaken) that first you use vmalloc to allocate a contiguous virtual memory region and suply a `nopage' method (via mmap) which returns the physical page coresponding to user's _and_ the module's virtual address plane; that means that after mucking about with page tables a while; you have two virtual contiguous memory regions (one user/one kernel) that both access the same physical scattered pages. ... ( ?? hmmm ??)
Andi > 2.3 and some patched 2.2 kernels also offer a way to do this Andi > directly (usion kiovecs and map_user_kiobuf()). This is not in Andi > standard 2.2 kernel though.
This approach basicly save's me from the `nopage' aspect of the afore mentioned method; but I dont have a contiguous memory region in kernel space; only in user land.
Linus > Basically, the way kio buffers work is that Linus > they are 100% based on only physical pages. There are no virtual Linus > issues at all in the IO, and that's exactly how I want it. There Linus > is no reason to confuse virtual addresses into this, because the Linus > thing should be usable even in the complete absense of virtual Linus > mappings (ie the kernel can do direct IO purely based on pages - Linus > think sendfile() etc).
After reading that (above quoted from...): (http://www.ussg.iu.edu/hypermail/linux/kernel/0010.2/0338.html) I can understand why.
So If I have a collection of physical pages in a kio buffer is there a way to create a contigous virtual memory region out of that ?
ie: unsigned long kmap_kiovec(int nr, struct kiobuf *iovec[]);
Must it be done by modifying the page tables by hand ? (If so; is "linux" interrested in such an api as kmap_kiovec or is it total nonsence ?)
Is there a preferred way to do this (mmap -> nopage vs. map_user_kiobuf()) ?
Best regards, -Tristan
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