The GPIO controllers connected to the emulated PCIe bus via vhost-user can't generate interrupts.
Description of problem:
The problem is related to emulation of GPIO controllers using the vhost-user protocol for GPIO. The problem was detected when using the [vhost-device-gpio](https://github.com/rust-vmm/vhost-device) software. I have described the whole issue in https://github.com/rust-vmm/vhost-device/issues/613 , but it is QEMU related, and therefore I describe it here as well.
The broader context is described in https://stackoverflow.com/questions/75906208/how-to-connect-via-virtio-gui-running-on-host-with-gpio-in-a-qemu-emulated-virtu .
Steps to reproduce:
1. For Debian/testing you need to compile a libgpiod-2.1.1 (I assume that the following is done in the home directory directory of the `dev` user: `/home/dev`):

    ```
    wget https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/snapshot/libgpiod-2.1.tar.gz ; \
    tar -xzf libgpiod-2.1.tar.gz ; \
    cd libgpiod-2.1 ; \
    autoupdate ; \
    ./autogen.sh ; \
    make
    ```
 2. Download the vhost-device-gpio (`git clone https://github.com/rust-vmm/vhost-device.git`)
 3. Build the vhost-device-gpio (in the `vhost-device-gpio` subdirectory)

    ```
    export PATH_TO_LIBGPIOD=/home/dev/libgpiod-2.1
    export SYSTEM_DEPS_LIBGPIOD_NO_PKG_CONFIG=1
    export SYSTEM_DEPS_LIBGPIOD_SEARCH_NATIVE="${PATH_TO_LIBGPIOD}/lib/.libs/"
    export SYSTEM_DEPS_LIBGPIOD_LIB=gpiod
    export SYSTEM_DEPS_LIBGPIOD_INCLUDE="${PATH_TO_LIBGPIOD}/include/"
    cargo build --features "mock_gpio"
    ```
 4. Start vhost-device-gpio: (`LD_LIBRARY_PATH=/home/emb/libgpiod-2.1/lib/.libs/ ./vhost-device-gpio -s /tmp/gpio.sock -l s4`)
 5. Download the Buildroot 2023.11.1 (`wget https://buildroot.org/downloads/buildroot-2023.11.1.tar.xz` in another directory) and unpack it. Buildroot and the main directory of Buildroot tree are denoted by BR if the following description.
 6. Configure BR (run `make qemu_aarch64_virt_defconfig` in the main BR directory, run `make menuconfig` and select external toolchain, `BR2_PACKAGE_LIBGPIOD=y`, `BR2_PACKAGE_LIBGPIOD_TOOLS=y`, run `make linux-menuconfig` and select `CONFIG_GPIO_VIRTIO=m` in the kernel configuration)
 7. Build the Linux and QEMU (run `make` in the BR directory).
 8. Run the emulation in BR/output/images, using the command line given above.
 9. After the virtual machine starts, log in as root and load the driver: `modprobe gpio-virtio`
10. Try to monitor changes of one of the emulated pins: `gpiomon 0 0`
11. You'll get the error message:

    ```
    gpiomon: error waiting for events: No such device
    ```
Additional information:
[0009-enable-F-IRQ-in-virtio-pci.patch](/uploads/39bc04b2d94063ccd539c5cfbc9cd105/0009-enable-F-IRQ-in-virtio-pci.patch)