LPC UART device pause/resume could be better

[jordanhendricks]

The Entity trait provides no-op pause and resume default methods that can optionally be implemented by entities. These methods are used by the migration code to pause devices before migrating their state and resume them on the target.

It is possible for the LPC UART device to continue receiving data after devices are paused, as a user might be connected to the console and typing. To allow for a consistent snapshot of device state during migration, the LPC UART should implement pause and resume methods.

[hawkw]

Am I correct that this should have been closed by #411?

[jordanhendricks]

#411 took a pass at a cheap version of this interface (to ensure that the serial console wouldn't hang after live migration, #391).

propolis/lib/propolis/src/hw/uart/lpc.rs

Lines 23 to 25 in 1ae1ee3

	// In the absence of better interfaces for chardev save/restore behavior,
	// allow the device to be coarsely paused (dropping all reads and writes).
	paused: bool,

captures (albeit tersely) what a more complete interface would look like.

That said, I'm not sure how critical a more complete pause/resume implementation for the UART is; afaik we haven't noticed further problems using the console across migrations, though I think we also haven't been testing migration as much as when I first came across #391.

[hawkw]

Updating the issue to reflect that this device does implement pause/resume now, but we could probably make it nicer.