Add alpine pause and alpine resume

[maxzinkus]

Support for pausing/unpausing instances using SIGSTOP and SIGCONT

• alpine pause and alpine resume commands
• show Paused in alpine list
• logic in start, stop, and ssh to keep things coherent

[maxzinkus]

Note that a paused VM will still take system resources, particularly, RAM.

[idroz]

I've been looking into this recently, particularly at VM suspension, but haven't considered SIGSTOP approach.

My current thinking was to hook up to the QMP socket (socket file is created for each running VM inside VM's subdirectory) and issue a Stop command, but not sure if it's an overkill, particularly if it won't free up resources (need to investigate).

I'm not familiar enough with SIGSTOP. What would be the main use case here and are there any downsides?

[maxzinkus]

SIGSTOP suspends a process, stopping it from being scheduled onto the CPU. It keeps around all the resources (process table entry, VMM, etc) for immediate resumption with SIGCONT.

The guest will resume thinking no time has passed, so this could contribute to clock desync. We could throw a machineConfig.Exec("hwclock -s") in the resume flow?

In my mind, the use case is to free up host CPU cycles and power consumption without having to fully shut down.

[idroz]

Cool - I think resume should have machineConfig.Exec("hwclock -s"), similar to alpine start. Would make it more seamless for the user

[maxzinkus]

Notably, (and I think this is kind of nice), it does not release the port bindings for hostfwd. That means we won't have weird bind crashes upon resume.

[idroz]

Agreed - was playing around with the PR and spotted that - it is quite nice I think.

[maxzinkus]

Added the hwclock-resync, again with the assumption that the system time is the user's responsibility

[idroz]

Nice - could you also run go run document.go inside /docs directory and commit changes into this PR - it'll update the main docs with the new CLI commands.

[maxzinkus]

Done! And thanks, I can do that in the future for PRs now that I know how :)