Strange "root on %s" automatic selection

[Joel CARNAT <joel%carnat.net@localhost>]

Hi,

I have NetBSD 4.99.3 installed on my Shuttle.
It has an IDE disk driver (wd0, where NetBSD is installed) and two card
readers (sd0 and sd1, that are empty).

With the NetBSD/i386 kernel, the boot process detects "root on wd0a".
With the NetBSD/xen kernel, the boot process don't automatically detect
"root on wd0a" and even propose a default "root on sd0a".

########################################################################
<...>
umass0 at uhub2 port 7 configuration 1 interface 0
umass0: Ltd Winter Ver1.3, rev 2.00/0.05, addr 2
umass0: using SCSI over Bulk-Only
scsibus0 at umass0: 2 targets, 2 luns per target
sd0 at scsibus0 target 0 lun 0: <USB2.0, CardReader CF RW, 0.0>> disk
removable
sd0: drive offline
sd1 at scsibus0 target 0 lun 1: <USB2.0, CardReader Combo, 0.0>> disk
removable
sd1: drive offline
wd0 at atabus0 drive 0: <ST3120026A>
wd0: drive supports 16-sector PIO transfers, LBA48 addressing
wd0: 111 GB, 232581 cyl, 16 head, 63 sec, 512 bytes/sect x 234441648
sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
wd0(viaide0:0:0): using PIO mode 4, Ultra-DMA mode 5 (Ultra/100) (using
DMA)
atapibus0 at atabus1: 2 targets
<..>
sd0(umass0:0:0:0):  Check Condition on CDB: 0x00 00 00 00 00 00
    SENSE KEY:  Not Ready
     ASC/ASCQ:  Medium Not Present

sd1(umass0:0:0:1):  Check Condition on CDB: 0x00 20 00 00 00 00
    SENSE KEY:  Not Ready
     ASC/ASCQ:  Medium Not Present

boot device: sd0
root on sd0a dumps on sd0b
sd0(umass0:0:0:0):  Check Condition on CDB: 0x00 00 00 00 00 00
    SENSE KEY:  Not Ready
     ASC/ASCQ:  Medium Not Present

sd0(umass0:0:0:0):  Check Condition on CDB: 0x00 00 00 00 00 00
    SENSE KEY:  Not Ready
     ASC/ASCQ:  Medium Not Present

vfs_mountroot: can't open root device
cannot mount root, error = 19
root device (default sd0a): AT S7=45 S0=0 L1 V1 X4 &c1 E1 Q0
use one of: nfe0 fxp0 fxp1 fwip0 sd0[a-p] sd1[a-p] wd0[a-p] cd0[a-p] ddb
halt reboot
root device (default sd0a): wd0a                          
dump device (default wd0b):                               
file system (default generic): 
root on wd0a dumps on wd0b
mountroot: trying lfs...
mountroot: trying ffs...
root file system type: ffs
init path (default /sbin/init): 
init: copying out path `/sbin/init' 11
Wed Nov 29 23:35:19 CET 2006
########################################################################

Manually entering "wd0a" boots Xen correctly.
Is there a specific parameter to give to grub so that the root is
correctly automatically detected ? Or do I have to force it in the
kernel file ?

My Grub conf boot part is
########################################################################
title   NetBSD/xen (serial)
        root (hd0,0,a)
        kernel (hd0,0,a)/usr/pkg/xen3-kernel/xen.gz dom0_mem=196608 
com1=9600,8n1
        module (hd0,0,a)/netbsd-xen0 root=/dev/hda1 ro console=ttyS0
########################################################################

TIA,
        Jo