UIO Devices and user processes

[Yann Droneaud]

Le mardi 06 octobre 2015 à 10:13 -0400, Kenneth Adam Miller a écrit :
> 
> 
> On Tue, Oct 6, 2015 at 9:58 AM, Yann Droneaud <ydroneaud at opteya.com>
> wrote:
> > Le mardi 06 octobre 2015 à 09:26 -0400, Kenneth Adam Miller a écrit
> > :
> > 
> > > Any body know about the issue of assigning a process a region of
> > > physical memory to use for it's malloc and free? I'd like to just
> > > have the process call through to a UIO driver with an ioctl, and
> > then
> > > once that's done it gets all it's memory from a specific region.
> > >
> > 
> > You mean CONFIG_UIO_DMEM_GENIRQ (drivers/uio/uio_dmem_genirq.c)
> > 
> > See:
> > http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/comm
> > it/?id=0a0c3b5a24bd802b1ebbf99e0b01296647b8199b
> > http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/comm
> > it/?id=b533a83008c3fb4983c1213276790cacd39b518f
> > https://www.kernel.org/doc/htmldocs/uio-howto/using-uio_dmem_genirq
> > .html
> > 
> > 
> Well I don't think that does exactly what I would like, although I've
> got that on my machine and I've been compiling it and learning from
> it. Here's my understanding of the process of the way mmap works:
> 
> Mmap is called from userland and it maps a region of memory of a
> certain size according to the parameters given to it, and the return
> value it has is the address at which the block requested starts, if
> it was successful (which I'm not addressing the unsuccessful case
> here for brevity). The userland process now has only a pointer to a
> region of space, as if they had allocated something with new or
> malloc. Further calls to new or malloc don't mean that the pointers
> returned will preside within the new mmap'd chunk, they are just
> separate regions also mapped into the process. 
> 

You have to write your own custom allocator using the mmap()'ed memory
your retrieved from UIO.

> What I would like is a region of memory that, once mapped to a
> process, further calls to new/malloc return pointers that preside
> within this chunk. Calls to new/malloc and delete/free only edit the
> process's internal table, which is fine. 
> 
> Is that wrong? Or is it that mmap already does the latter?

It's likely wrong. glibc's malloc() using brk() and mmap() to allocate
anonymous pages. Tricking this implementation to use another mean to
retrieve memory is left to the reader.

Anyway, are you sure you want any random calls to malloc() (from glibc
itself or any other linked-in libraries) to eat UIO allocated buffers ?
I don't think so: such physically contiguous, cache coherent buffers
are premium ressources, you don't want to distribute them gratuitously.

Regards.

-- 
Yann Droneaud
OPTEYA