Re: Xen Unikernel: from zero to Hello World!

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 27/10/15 19:42, Carl Patenaude Poulin wrote:
> Hi all,
>
> Thanks to Andrew Cooper's priceless help, I've managed to put
> together a Xen PV kernel that does nothing except loop forever. I'm
> going to try making it print "Hello, world!".
>
> I was hoping someone could fact-check my research. What I've dug up
> is:
> * I need to load my unikernel with `xl -c` to get console output.

-c is a parameter to `xl create`

Alternatively, you could do `xl create` followed by `xl console` later. 
However, there are usability issues with both of these approaches in
combination with short-lived domains.

> * My kernel currently refuses to load with the `-c` option. From
> what I've read online, this is because I need to implement a console
> device driver, similar to Mini-OS's xencons_ring.c.

What is the error message you get?  A PV guest always has a console, so
there is no refusal available to happen.  I suspect you have actually
hit one of the usability issues I eluded to above.

> * On Xen debug builds, I don't need a console device driver, I can
> just do HYPERVISOR_console_io hypercalls.

Always do all development with a debug hypervisor.  Even for guests, you
get more useful information out when something goes wrong.

Longterm you will want a better way of getting logging out of the guest,
but console_io hypercalls are a good start for very early bits.

~Andrew