Xen PV Console notes
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                                                      Stefano Stabellini
                                        stefano.stabellini@eu.citrix.com


Xen traditionally provided a single pv console to pv guests, storing the
relevant information in xenstore under /local/domain/$DOMID/console.

Now many years after the introduction of the pv console we have
multiple pv consoles support for pv and hvm guests; multiple pv
console backends (qemu and xenconsoled) and emulated serial cards too.

This document tries to describe how the whole system works and how the
different components interact with each others.

The first PV console path in xenstore remains:

/local/domain/$DOMID/console

the other PV consoles follow the conventional xenstore device path and
live in:

/local/domain/$DOMID/device/console/$DEVID.

The output of a PV console, whether it should be a file, a pty, a
socket, or something else, is specified by the toolstack in the xenstore
node "output", under the relevant console section.
For example:

# xenstore-read /local/domain/26/device/console/1/output
pty

The backend chosen for a particular console is specified by the
toolstack in the "type" node on xenstore, under the relevant console
section.
For example:

# xenstore-read /local/domain/26/console/type
xenconsoled

The supported values are only xenconsoled or ioemu; xenconsoled has
several limitations: it can only be used for the first PV console and it
can only have a pty as output.

If the output is a pty, backends write the device name to the "tty" node
in xenstore under the relevant console path.

Emulated serials are provided by qemu-dm only to hvm guests; the number
of emulated serials depends on how many "-serial" command line options
are given to qemu. The output of a serial is specified as argument to
the -serial command line option to qemu. Qemu writes the tty name to
xenstore in the following path:

/local/domain/$DOMID/serial/$SERIAL_NUM/tty


xenconsole is the tool to connect to a PV console or an emulated serial
that has a pty as output. Xenconsole takes a domid as parameter plus an
optional console type (pv for PV consoles or serial for emulated
serials) and console number. Depending on the type and console
number, xenconsole will look for the tty node in different xenstore
paths, as described above.  If the user doesn't specify the console type
xenconsole will try to guess: if the guest is a pv guest it defaults to
PV console, if the guest is an hvm guest it defaults to emulated serial.

By default xl creates a pv console for hvm guests, plus an emulated
serial if the user specified 'serial = "pty"' in the VM config file.
Considering that xenconsole defaults to emulated serials for hvm guests,
executing xl create -c "domain" causes xenconsole to attach to the
emulated serial tty. This is most probably what the user wanted because
currently no bootloaders support xen pv consoles so the only way to
interact with a bootloader like grub over a console is to use the
emulated serial.
However the pv console is still easy to use with Linux PV on HVM guests:
the user just need to pass "console=hvc0" to the kernel command line and
then execute "xl console -t pv <domain>" to connect to it.

When using stubdoms the serial cards are still emulated by qemu (this
time running in the stubdom), the number of serial cards and where the
output goes is still specified using qemu command line options.
The difference is that for each emulated serial card there must be a pv
console connection between the stubdom and dom0 to export the serial
output from the stubdom to dom0. The pv console backend for stubdom's pv
consoles is always ioemu because multiple pv consoles support is a
requirement in this case, considering that minios has its own pv console
too. In order to simplify the setup when using stubdoms the hvm guest
can only have one pv console with xenstored as backend (the stubdom
could provide pv console backends to the hvm guest but then it would
need another pv console connection for each console backend to export
the pty to dom0).
