Re: better console support

[Joe Bonasera <joe.bonasera@sun.com>]

Keir Fraser wrote:
> On 25/1/07 17:00, "Joe Bonasera" <joe.bonasera@sun.com> wrote:
> 
>> There seems to be a some confusion over properly setting
>> up what Xen and dom0 use for console devices.
>>
>> Is anybody actively working on changes in this area?
> 
> I'm not sure there's any confusion. By default Xen uses the VGA console
> until dom0 starts executing, at which point it gives it up. At that point
> Xen has no console devices. If the user overrides any settings, for example
> to give Xen com1 then we make efforts to prevent dom0 from conflicting
> (e.g., by silently disallowing its attempts to mess with com1).
> 
>  -- Keir
> 

The silently disallowing thing, as well as the vga vs. vga all the time
is what is causing Solaris users confusion.

The standard internal testing setup inside Sun that most
people run with is debug Xen and debug dom0 both
sharing either the VGA console or a single serial port console.
We also see field support of the eventual product needing the
ability to interact with both consoles on the same physical
device.  So we want to make the dom0 just handle sharing the console
as automatically as possible.

The algorithm I'm envisioning is something like:
	hyper_cons_dev = ... /* from Xen capabilites, one of
				com1, com2, vga, or vgakeep */

	dom0_cons_dev = ... /* from dom0 command line, one of
			com1, com2, vga, or hypervisor --
			where hypervisor means use the
			HYPERVISOR_console_io() calls */

then in our dom0 startup we could do something like :

	if (hyper_cons_dev is vgakeep)
		if (dom0_cons_dev is vga)
			set dom0_cons_dev = hypervisor
		endif
	else if (hyper_cons_dev is vga)
		/* should just work */
	else if (hyper_cons_dev is same as dom0_cons_dev)
		set dom0_cons_dev = hypervisor
	endif

If that's a non-issue for Linux that's ok, it could
just ignore the new capability.

Joe