amazon/xen... any way at all to pass a message/signal/semaphoere/morse-code to the boot loader?

[Julian Elischer]

On 12/4/17 3:13 pm, Colin Percival wrote:
> On 04/11/17 23:04, Julian Elischer wrote:
>> On 12/4/17 12:34 pm, Colin Percival wrote:
>>> [CCing freebsd-cloud, which is the right place for discussions of FreeBSD/EC2]
>>>
>>> On 04/11/17 21:03, Julian Elischer wrote:
>>>> In Amazon ec2 they have no console access (though I heard rumors that it was
>>>> available I have not seen any sign of it) so I'd like to put a "recovery
>>>> partition" into an AMI.
>>>> The trick is how to convince it to boot to that instead of the regular action.
>>> Can you get what you want via gptboot's support for selecting the partition
>>> to boot via "bootonce" and "bootme" flags?
>> not if you can't get onto to the machine.
> Well, I meant that you'd set this up in advance, so that if it can't boot the
> normal partition it would automatically fall back to the recovery partition.
>
>>> Maybe you can repurpose some of the logic used for booting over NFS?  I've
>>> never heard of people booting over NFS when the initial bootstrap comes from
>>> disk rather than PXE, but I assume it's possible...?
>> Oh I've done it, in the past but you still have the same issue..
>> how do you signal the boot code to do this?
>>
>> (does an AMI have a bios capable of doing network operations?) I was thinking
>> about whether we could add a really simple xn driver into the bootcode to allow
>> us to have an console of sorts (accessible from an adjacent machine only??)
> Oh, good point.  No, the boot loader doesn't know anything about PV drivers,
> and there's no emulation, so you can't do anything networky from the boot
> loader.
>
I was thinking of a specialist AMI bootcode that could handle just an 
xn0 interface.

I don't know enough about the xen drivers to know if that would even 
be possible.