[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?

> The ideal thing would be if there was  way to 'influence' one of the smbios
> values in some way, and have the boot code see it, but I'm open to any
> suggestions.
> I really need only 1 bit of information to get through.
> Possibilties include "changing the VM to have only 2G of ram" (we'd never do
> that in a real machine).
> or maybe temporarily removing all the disks other than the root drive? Almost
> anything I could do to signal the boot code to behave differently.

I don't think adding/removing disks will be useful, since the extra disks will
be Xen blkfront devices; AFAIK the boot loader doesn't know anything about
these.  (The boot device is also a blkfront device but gets ATA emulation for
the benefit of boot loaders.)

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...?

Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
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