Re: amd64 panics w/XEN_DOM0, not GENERIC

[Todd Kover <kovert%omniscient.com@localhost>]

 > > I slowly paired things down so I was not running any xen daemons and
 > > consequently, no domUs, and had eventually dedicated all of the 4gb
 > > of memory to dom0.
 > 
 > Which is probably the cause of your problem.

I left out some detail on what I slowly paired down: I started running
xen daemons and domUs.  I got the kern_physio.c panic.  A few days
later after it was clear it was systemic, I tried just the xen daemons
with no domUs, got the kern_physio.c panic, after a few more days
of repititions, I killed the xen daemons and continue to get the
kern_physio.c panic.  I ran generic for a few days, didn't get any
panics, went back to xen kernel, didn't bother going back to starting
xen daemons and got a new panic.  I tried GENERIC kernels interpersed in
there and it was always fine.

Whether or not xen daemons running seems to have no impact.

However, in any case, it seems like a panic is never the right
behavior...
 
 > > RECENT CRASH:
 > > uvm_fault(0xffffffff80b24e80, 0xffffffff81400000, 1) -> e
 > > fatal page fault in supervisor mode
 > > trap type 6 code 0 rip ffffffff804bd443 cs e030 rflags 10282 cr2   
 > > ffffffff81
 > > 400140 cpl 0 rsp ffffa0004fb108d0
 > > kernel: page fault trap, code=0
 > > Stopped in pid 850.1 (chunker) at       netbsd:pmap_kenter_pa 
 > > +0x173:    movq 0
 > > (%rax),%rsi
 > > pmap_kenter_pa() at netbsd:pmap_kenter_pa+0x173
 > > ubc_alloc() at netbsd:ubc_alloc+0x25d
 > > ubc_uiomove() at netbsd:ubc_uiomove+0xba
 > > ffs_write() at netbsd:ffs_write+0x684
 > > VOP_WRITE() at netbsd:VOP_WRITE+0x2d
 > > vn_write() at netbsd:vn_write+0xce
 > > dofilewrite() at netbsd:dofilewrite+0x7f
 > > sys_write() at netbsd:sys_write+0x72
 > > syscall() at netbsd:syscall+0x98
 > 
 > That looks exactly like the stack trace I reported in PR port-xen/38699.

indeed it does.  The uvm_fault is a new one for me, but maybe the other
problem I was seeing was fixed exposing this one.  I'll try it with the
xen daemons running and see if it makes any difference and keep an eye
on your PR.

-Todd