Shared memory between guest and host in QEMU

I found the following explanation:

Shared memory IPC (inter-process communication) is a method of exchanging data between processes or virtual machines using a shared memory region. Qemu is a software that emulates various hardware architectures and allows running different operating systems on the same host machine.

One way to enable shared memory IPC in qemu is to use the ivshmem device, which is a PCI device that provides access to a shared memory region on the host. The ivshmem device can be configured to use either a POSIX shared memory file (such as /dev/shm/data.dat) or a memory backend object (such as memory-backend-file) as the source of the shared memory12.

To use the ivshmem device, you need to specify the following options when launching qemu:

-device ivshmem-plain,memdev=hostmem where hostmem is the name of the memory backend object that defines the size, path, and sharing mode of the shared memory region. -object memory-backend-file,size=...,share=on,mem-path=...,id=hostmem where you specify the size, path, and ID of the memory backend object. The share=on option is important to enable sharing the memory region with other processes or VMs. For example, the following command will create a 256 MB shared memory region backed by the file /dev/shm/data.dat and expose it to the guest VM as an ivshmem device:

qemu-system-x86_64 -device ivshmem-plain,memdev=hostmem -object memory-backend-file,size=256M,share=on,mem-path=/dev/shm/data.dat,id=hostmem ...

Note that the file /dev/shm/data.dat will not be visible on the guest VM, as it is only used by the host to allocate the shared memory region. The guest VM will see the shared memory region as a PCI device with a BAR (base address register) that maps to the shared memory address space. You can use the lspci command on the guest VM to verify that the ivshmem device is present and get its BAR address2.

To access the shared memory region from the guest VM, you need to use the mmap system call to map the BAR address to the guest’s virtual address space. For example, the following C code snippet will map the first 4 KB of the shared memory region to a pointer shm:

#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>

#define SHM_SIZE 4096 // size of the shared memory region to map
#define SHM_BAR 0xfebf0000 // BAR address of the ivshmem device (obtained from lspci)

int main() {
    int fd = open("/dev/mem", O_RDWR); // open the /dev/mem device file
    if (fd == -1) {
        perror("open");
        return -1;
    }
    void *shm = mmap(NULL, SHM_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, SHM_BAR); // map the BAR address to shm
    if (shm == MAP_FAILED) {
        perror("mmap");
        return -1;
    }
    // do something with shm, such as reading or writing data
    munmap(shm, SHM_SIZE); // unmap the shared memory region
    close(fd); // close the /dev/mem device file
    return 0;
}