getting physical address of buffer into user space

getting physical address of buffer into user space

[Marty Leisner]

I've read tutorials and seen example programs on how to get page information into userspace 
(eventually, I'll want to dma from a UIO driver)

For POC, I have a simple driver with an mmap rule, which does kmalloc on a new buffer.

The driver can read/write the memory, and the user space program can read/write the memory (and both
sides agree on the data read/written).

It appears in /proc/self/maps, and when I try to look it up in /proc/self/pagemap, it says its
neither swapped nor present.

This seems like a clever way -- if it works (the alternative is sloppy ioctl's or other custom
methods)

-- 
Marty Leisner <linux@rochester.rr.com>

Re: getting physical address of buffer into user space

[Alexander Duyck]

On Tue, Feb 18, 2020 at 12:56 PM Marty Leisner <linux@rochester.rr.com> wrote:
>
> I've read tutorials and seen example programs on how to get page information into userspace
> (eventually, I'll want to dma from a UIO driver)

Instead of using a UIO driver you may want to consider a VFIO driver
as you may not be able to access memory directly from the device with
a UIO driver if an IOMMU is present in the system. UIO is normally
only used for small embedded controllers where you just need MMIO
access.

> For POC, I have a simple driver with an mmap rule, which does kmalloc on a new buffer.
>
> The driver can read/write the memory, and the user space program can read/write the memory (and both
> sides agree on the data read/written).
>
> It appears in /proc/self/maps, and when I try to look it up in /proc/self/pagemap, it says its
> neither swapped nor present.
>
> This seems like a clever way -- if it works (the alternative is sloppy ioctl's or other custom
> methods)

I suggest you just take a look at VFIO. You can find some user
documentation here:
https://www.kernel.org/doc/html/latest/driver-api/vfio.html

There are multiple userspace drivers out there that make use of UIO or
VFIO functionality. If you are in need of examples just go take a look
at the DPDK project (http://git.dpdk.org/dpdk/) as they have all of
their NIC drivers implemented in userspace and are using either a
igb_uio driver or vfio driver to connect to them.