freebsd stable - Oct 2018 - How to boot from GPT partition without "bootme" attribute?

I have disk with GPT scheme and three partitions:

p1 - freebsd-boot
p2 - freebsd-ufs
p3 - freebsd-ufs

 pmbr is installed on this disk, and gptboot is installed on p1. Both p2
and p3 contains valid FreeBSD installation, with /boot/loader, kernel,
and everything.

 I have attribute "bootme" set on p3, but not on p2.

 What should I do to boot from p2?

 I've tried to interrupt gptboot and override its choice:

 0:ad(0p3)/boot/loader

 with

 0:ad(0p2)/boot/loader

 After that loader, loaded from p2, loads kernel from p3 and boots
system from p3!

 If I have MBR, I could override "active" slice in boot0 MBR loader
interactively.

 Is it analogous feature for GPT?

-- 
// Lev Serebryakov

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On Tue, 2018-10-30 at 21:37 +0300, Lev Serebryakov
wrote:
> ?I have disk with GPT scheme and three partitions:
> 
> p1 - freebsd-boot
> p2 - freebsd-ufs
> p3 - freebsd-ufs
> 
> ?pmbr is installed on this disk, and gptboot is installed on p1. Both
> p2
> and p3 contains valid FreeBSD installation, with /boot/loader,
> kernel,
> and everything.
> 
> ?I have attribute "bootme" set on p3, but not on p2.
> 
> ?What should I do to boot from p2?
> 
> ?I've tried to interrupt gptboot and override its choice:
> 
> ?0:ad(0p3)/boot/loader
> 
> ?with
> 
> ?0:ad(0p2)/boot/loader
> 
> ?After that loader, loaded from p2, loads kernel from p3 and boots
> system from p3!
> 
> ?If I have MBR, I could override "active" slice in boot0 MBR
loader
> interactively.
> 
> ?Is it analogous feature for GPT?
> 
While loader(8) is loading the kernel, interrupt it to get the console
prompt (or ask the menu to give the prompt if you use menus) and do:

?unload
?set currdev=disk0p2
?boot

-- Ian