[Bug 299244] [PATCH] virtio_console ignores RX flow control, drops data

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=299244

            Bug ID: 299244
           Summary: [PATCH] virtio_console ignores RX flow control, drops
                    data
           Product: Base System
           Version: 15.1-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: equinox@diac24.net

Created attachment 275515
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=275515&action=edit
hacky patch

The virtio_console driver just completely ignores the return value from the tty
layer and keeps piling on data.  Needless to say, this doesn't work
particularly well for any nontrivial amount of data getting shoveled in and
gives a rather irritating admin experience. ("slow paste" in terminal clients,
anyone?)

Testing by dumping a 4MB file into a virtio console, on average I got 30
kilobytes (!!!) to arrive.  (Part of the problem there is that the
virtio_console driver *also* doesn't use the bypass path, delivering a byte at
a time instead.)

I've attached a hacky, "unclean" patch.  It seems to work reliably for me but I
might have overlooked something.  (In particular, not sure about cleaning up
hiwat_buf at device shutdown/destroy.)

I also have no idea how contributing patches to FreeBSD works; if anyone wants
to take this off my hands that'd be much appreciated.

Patch is on top of 21f25cc80de2f9f35d7698bf355b0353da45dbc7 (stable/15).

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