kern/165418: [xen] Problems mounting root filesystem from XENHVM

[Mike Kaganski]

The following reply was made to PR kern/165418; it has been noted by GNATS.

From: Mike Kaganski <mikekaganski at mail.ru>
To: bug-followup at FreeBSD.org, stefan.witzel at zvw.uni-goettingen.de
Cc:  
Subject: Re: kern/165418: [xen] Problems mounting root filesystem from XENHVM
Date: Tue, 27 Mar 2012 10:00:04 +1100

  > The Xen bootloader has problems with UFS2.
  > I'm only running PV guests, but pygrub doesn't recognise
  >UFS2 (and had to be patched for UFS). HVMs may have a similar issue
 
 Just to clarify things:
 
 1. To boot a VM, *XEN* needs to start its kernel. In case of HVM, its 
 job is simple: it considers the (virtual) boot disk as an opaque 
 bytestream with a boot record in proper place, and it's the boot 
 record's job to find and launch the kernel. In PV VMs, XEN needs to 
 launch the kernel as its "native" program, so it needs to find the 
 kernel file(s). Thus, either the kernel needs to be on a filesystem 
 recognizable by XEN, or the kernel file(s) must be copied directly to dom0.
 2. To work with a filesystem, the *VM kernel* needs to recognize it, 
 thus its drivers must be accessible to the kernel before it starts using 
 it. Thus, they need either to be compiled into the kernel itself, or to 
 be placed somewhere on a different filesystem (and the kernel needs to 
 know where to look for them).
 
 Generally, there is no need to patch XEN loader to use any new 
 filesystem in case of PV domUs. The patching way may eventually lead to 
 problems due to possible incompatibilities in different patches. It's 
 better to just copy the properly compiled (with the required filesystem 
 drivers compiled in) PV kernel to dom0 and configure its XEN startup 
 parameters.
 
 The OP indeed has the HVM FreeBSD kernel started OK, but its 
 configuration points to continue loading from a non-accessible device. 
 It's not a "xen bootloader" issue.
 
 --
 Best regards,
 Mike.