virtio-mem: introduce VIRTIO_MEM_F_PERSISTENT_SUSPEND

[davidhildenbrand]

virtio-mem in upstream QEMU and Linux support a new feature called VIRTIO_MEM_F_PERSISTENT_SUSPEND.

If provided by the device, the driver can safely allow the system to go into suspend mode (e.g., S5) without risking that the device will lose its state during the wakeup.

Quoting from the current proposal:

Before, the behavior while suspending to a deep sleep state and waking
up was not specified. For example, in x86 QEMU VM all devices receive a
reset request during wake-up. This would lead to unplugging of all the
plugged memory blocks. Due to this, suspending is disallowed in the
Linux Kernel, when plugged memory is present and
VIRTIO_MEM_F_PERSISTENT_SUSPEND feature flag is not advertised by the
virtio-mem device.

This new flag should signal to the guest driver, that the device can correctly suspend to a deep sleep state and then wake up without disrupting the plugged memory blocks.

Latest proposal: https://lkml.kernel.org/r/[email protected]

[mstsirkin]

BALLOT CREATED AT URL: https://groups.oasis-open.org/higherlogic/ws/public/ballot?id=3856