Date: Thu, 13 Nov 2025 12:20:22 +0100 From: =?UTF-8?Q?Bojan_Novkovi=C4=87?= <bnovkov@freebsd.org> To: freebsd-virtualization@freebsd.org Cc: Mark Johnston <markj@freebsd.org> Subject: RFC: transient virtual machines and "monitor mode" Message-ID: <9d28570c-0700-4d24-8977-6f5eb68c23eb@freebsd.org>
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Hi, As part of the work towards making unprivileged bhyve viable I made some changes to the VM lifecycle, so I'd like to get some feedback on the new VM lifecycle scheme. The first big change was to add mechanisms to automatically reclaim a virtual machine's resources when the owning process goes away, especially when it's killed by the OOM killer. To address this we decided to tie a virtual machine's lifecycle to a file descriptor. vmmctl's VMMCTL_VM_CREATE ioctl was extended with a new flag (VMMCTL_CREATE_DESTROY_ON_CLOSE) that instructs it to destroy all virtual machines associated with a specific vmmctl file descriptor when it is closed. vmmctl uses the 'devfs_xxx_cdevpriv' functions to maintain a per-descriptor list of such virtual machines. Under this regime the virtual machine gets destroyed once the owning process dies, which is not very useful when one just wants to restart a virtual machine. The second big change addresses this by introducing "monitor mode" to bhyve (for lack of a better term), which is enabled by passing the '-M' flag. This will instruct bhyve to create a transient virtual machine using the mechanism described in the above paragraph. At a certain point during startup, it will then fork itself and let the child process continue with the setup and start the VM. The parent process waits for the virtual machine to exit and will automatically restart it if the child exited with a "reboot" status (i.e., 0). Exiting with any other status code or killing the parent process will automatically destroy the VM. Monitor mode also works with non-transient VMs, but it will not destroy them when bhyve exits. Please feel free to leave comments on any of the following reviews: - https://reviews.freebsd.org/D53729 (vmm: Add ability to destroy VMs on close) - https://reviews.freebsd.org/D53731 (bhyve: Introduce monitor mode) Thanks, Bojan
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