Re: port-xen/59451: XEN3_DOM0 kernel finds the wrong root device

["Chuck Zmudzinski via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR port-xen/59451; it has been noted by GNATS.

From: Chuck Zmudzinski <frchuckz%gmail.com@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: port-xen/59451: XEN3_DOM0 kernel finds the wrong root device
Date: Mon, 2 Jun 2025 12:48:30 -0400

 On 6/2/2025 8:20 AM, Michael van Elst via gnats wrote:
 > The following reply was made to PR port-xen/59451; it has been noted by GNATS.
 > 
 > From: mlelstv%serpens.de@localhost (Michael van Elst)
 > To: gnats-bugs%netbsd.org@localhost
 > Cc: 
 > Subject: Re: port-xen/59451: XEN3_DOM0 kernel finds the wrong root device
 > Date: Mon, 2 Jun 2025 12:18:41 -0000 (UTC)
 > 
 >  gnats-admin%NetBSD.org@localhost ("Chuck Zmudzinski via gnats") writes:
 >  ...
 >  > My testing verified that Manuel is correct: bootdev= and root= behave the same way,
 >  > but actually root= is better because it gives me three more characters for the
 >  > boot command line before it overflows and starts to truncate the end of the command
 >  > line.
 >  
 >  
 >  bootdev= and root= currently work the same way.
 >  
 >  The supplied string is checked against all disk and network interface
 >  device names.
 >  
 >  If there is a match, then the result is a pointer "booted_device"
 >  to the driver and an integer "booted_partition". The kernel will
 >  later try to access the particular driver and read data from the
 >  numbered partition. Obviously the partition is ignored for a
 >  network interface, and it should be 0 for something like a wedge
 >  (that doesn't have partitions).
 >  
 >  If there is no match, then the string is passed as is, similar to
 >  a hardcoded embedded "config root" in the kernel config file. The
 >  result is the string pointer "bootspec" that is evaluated later
 >  by the kernel.
 >  
 >  In either case, the supplied string can have 144 bytes (including
 >  the terminating NUL character), but the parser truncates the
 >  full command line to 255 characters first, not sure why.
 >  
 
 After further investigation...
 
 I don't think the string passed by the bootdev= setting is overflowing
 in the kernel. Rather, I think the length of the arguments of the
 "load" command that the bootloader uses to load the DOM0 kernel image
 is overflowing. I think so because I was actually able to use a
 long UUID as the wedgename by reducing the number of characters in
 the filename of the kernel I am booting, and reducing that helps
 reduce the length of the arguments to the bootlader's "load" command.
 
 In any case, I decided to set a label on the GPT partition like "netsbd-root"
 instead of using the long UUID as the wedgename to help ensure that nothing
 in my boot.cfg gets truncated.