uio

root@gobgp:~# lspci | grep -i ether
01:00.0 Ethernet controller: Red Hat, Inc. Virtio network device (rev 01)
07:00.0 Ethernet controller: Red Hat, Inc. Virtio network device (rev 01)
root@gobgp:~# lspci -s 07:00.0
07:00.0 Ethernet controller: Red Hat, Inc. Virtio network device (rev 01)
root@gobgp:~# lspci -s 07:00.0 -v
07:00.0 Ethernet controller: Red Hat, Inc. Virtio network device (rev 01)
        Subsystem: Red Hat, Inc. Virtio network device
        Physical Slot: 0-6
        Flags: bus master, fast devsel, latency 0, IRQ 38
        Memory at 10c40000 (32-bit, non-prefetchable) [size=4K]
        Memory at 8000c00000 (64-bit, prefetchable) [size=16K]
        Expansion ROM at 10c00000 [disabled] [size=256K]
        Capabilities: [dc] MSI-X: Enable- Count=3 Masked-
        Capabilities: [c8] Vendor Specific Information: VirtIO: <unknown>
        Capabilities: [b4] Vendor Specific Information: VirtIO: Notify
        Capabilities: [a4] Vendor Specific Information: VirtIO: DeviceCfg
        Capabilities: [94] Vendor Specific Information: VirtIO: ISR
        Capabilities: [84] Vendor Specific Information: VirtIO: CommonCfg
        Capabilities: [7c] Power Management version 3
        Capabilities: [40] Express Endpoint, MSI 00
        Kernel driver in use: uio_pci_generic

root@gobgp:~# 

root@gobgp:~# cat /sys/bus/pci/drivers/uio_pci_generic/bind
cat: /sys/bus/pci/drivers/uio_pci_generic/bind: Permission denied
 
root@gobgp:~# ls -l /sys/bus/pci/devices/0000\:07\:00.0
lrwxrwxrwx 1 root root 0 Aug 24 14:20 /sys/bus/pci/devices/0000:07:00.0 -> ../../../devices/pci0000:00/0000:00:01.6/0000:07:00.0
root@gobgp:~# ls -l /sys/bus/pci/devices/0000\:07\:00.0/driver 
lrwxrwxrwx 1 root root 0 Aug 24 14:20 /sys/bus/pci/devices/0000:07:00.0/driver -> ../../../../bus/pci/drivers/uio_pci_generic
root@gobgp:~#

root@gobgp:~# cat /dev/uio0
cat: /dev/uio0: Invalid argument
root@gobgp:~# 

Example code using uio_pci_generic
Here is some sample userspace driver code using uio_pci_generic:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

int main()
{
    int uiofd;
    int configfd;
    int err;
    int i;
    unsigned icount;
    unsigned char command_high;

    uiofd = open("/dev/uio0", O_RDONLY);
    if (uiofd < 0) {
        perror("uio open:");
        return errno;
    }
    configfd = open("/sys/class/uio/uio0/device/config", O_RDWR);
    if (configfd < 0) {
        perror("config open:");
        return errno;
    }

    /* Read and cache command value */
    err = pread(configfd, &command_high, 1, 5);
    if (err != 1) {
        perror("command config read:");
        return errno;
    }
    command_high &= ~0x4;

    for(i = 0;; ++i) {
        /* Print out a message, for debugging. */
        if (i == 0)
            fprintf(stderr, "Started uio test driver.\n");
        else
            fprintf(stderr, "Interrupts: %d\n", icount);

        /****************************************/
        /* Here we got an interrupt from the
           device. Do something to it. */
        /****************************************/

        /* Re-enable interrupts. */
        err = pwrite(configfd, &command_high, 1, 5);
        if (err != 1) {
            perror("config write:");
            break;
        }

        /* Wait for next interrupt. */
        err = read(uiofd, &icount, 4);
        if (err != 4) {
            perror("uio read:");
            break;
        }

    }
    return errno;
}