ZFS corruption using zvols as backingstore for hvm VM's

[Brian Buhrow]

	hello Roger.  thanks for engaging with me on this issue.  I think
I've made progress on the issue and have a better handle on what's going
wrong.  There seem to be a cascade of bugs here, which I'll try to
enumerate.  

1.  The disk corruption issue seems to be a bug in qemu whereby the
emulated IDE disk controller issues partial writes instead of full writes
or no writes with appropriate failure to the disk.  The IDE driver in
NetBSD-5.2 doesn't play well with this behavior, in fact, NetBSD until May
of 2020, doesn't play well with this behavior
See: 
http://mail-index.NetBSD.org/source-changes/2020/05/24/msg117668.html

2.  This causes memory corruption in the OS itself, which can trigger a xen
server crash!  (In my view, no matter how badly behaved the guest OS is, it
shouldn't be able to bring down the xen server.)

3.  Running NetBSD-5.2/i386 as a domu, which works flawlessly under xen3, gets a 
panic: HYPERVISOR_mmu_update failed

	I suspect this can be worked around using some of the command line
options under Xen, xpti=true,domu=false, perhaps?  
Are there others I should consider?
	If I can get the domu kernel working, or can back port the patch
listed  in bug 1 above, I should be off to the races.  Still, I think
there's a serious issue here in bug 2, listed above that ought to be looked
at.  Unfortunately, I don't hav a way to readily reproduce it.

Any thoughts on how to achieve pv32 backward compatibility with xen3 would be
greatly appreciated.

Pv64 NetBSD-5.2 seems to work fine.

-thanks
-Brian