HEAD Xen test fails with panic

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
for some time, NetBSD/Xen anita tests fails with this panic:
panic: kernel diagnostic assertion "usec >= 0 && usec < 1000000" failed: file 
"/home/builds/ab/HEAD/src/sys/kern/subr_time.c", line 92 
cpu3: Begin traceback...
vpanic(c04b3a50,cd613c00,cd613c1c,c0390291,c04b3a50,c04b3a91,c04e1abe,c04e1a90,5c,0)
 at netbsd:vpanic+0x117
kern_assert(c04b3a50,c04b3a91,c04e1abe,c04e1a90,5c,0,ea6c2b7b,ffffffff,cd613c34,c0390346)
 at netbsd:kern_assert+0x23
tvtohz(cd613c28,c0bf24e1,0,fffa79e7,cd613c9c,c026efec,cd613c68,cd613c74,2,0) at 
netbsd:tvtohz+0xf1
tstohz(cd613c68,cd613c74,2,0,cd613c80,51c73505,cd613c68,0,0,124df7c) at 
netbsd:tstohz+0x36
nanosleep1(c12df2a0,3,1,cd613cb8,0,0,0,989680,fffffffe,c045bec5) at 
netbsd:nanosleep1+0xac
sys_clock_nanosleep(c12df2a0,cd613d00,cd613d28,c0eb3000,c0e83c24,1dd,c14c8bec,3,1,ba9fb934)
 at netbsd:sys_clock_nanosleep+0x5d
syscall() at netbsd:syscall+0x83

(whole logs at http://www-soc.lip6.fr/~bouyer/NetBSD-tests/xen/HEAD/)

It's still there in the 2013-05-12 13:00 UTC sources.
It did first show up with the 2013-04-28 15:40 UTC sources.

Any idea what could cause this ?

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--