[RFC 0/3] add snapshot/restore fuzzing device

[Richard Liu <richy.liu.2002@gmail.com>]

This RFC adds a virtual device for snapshot/restores within QEMU. I am working
on this as a part of QEMU Google Summer of Code 2022. Fast snapshot/restores
within QEMU is helpful for code fuzzing.

I reused the migration code for saving and restoring virtual device and CPU
state. As for the RAM, I am using a simple COW mmaped file to do restores.

The loadvm migration function I used for doing restores only worked after I
called it from a qemu_bh. I'm not sure if I should run the migration code in a
separate thread (see patch 3), since currently it is running as a part of the
device code in the vCPU thread.

This is a rough first revision and feedback on the cpu and device state restores
is appreciated.

To test locally, boot up any linux distro. I used the following C file to
interact with the PCI snapshot device:

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

    int main() {
        int fd = open("/sys/bus/pci/devices/0000:00:04.0/resource0", O_RDWR | O_SYNC);
        size_t size = 1024 * 1024;
        uint32_t* memory = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

        printf("%x\n", memory[0]);

        int a = 0;
        memory[0] = 0x101; // save snapshot
        printf("before: value of a = %d\n", a);
        a = 1;
        printf("middle: value of a = %d\n", a);
        memory[0] = 0x102; // load snapshot
        printf("after: value of a = %d\n", a);

        return 0;
    }

Richard Liu (3):
  create skeleton snapshot device and add docs
  implement ram save/restore
  use migration code for cpu and device save/restore

 docs/devel/snapshot.rst |  26 +++++++
 hw/i386/Kconfig         |   1 +
 hw/misc/Kconfig         |   3 +
 hw/misc/meson.build     |   1 +
 hw/misc/snapshot.c      | 164 ++++++++++++++++++++++++++++++++++++++++
 migration/savevm.c      |  84 ++++++++++++++++++++
 migration/savevm.h      |   3 +
 7 files changed, 282 insertions(+)
 create mode 100644 docs/devel/snapshot.rst
 create mode 100644 hw/misc/snapshot.c

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