Re: trouble running FreeBSD with Xen-4.11 (including weird performance issues)

[Brian Buhrow <buhrow%nfbcal.org@localhost>]

	hello.  Although I'm not running NetBSD-9.99.xx as a dom0, I wonder if this problem has
anything to do with the maxphys limitations in NetBSD being limited to 64k, but FreeBSD having
a different maxphys limitation in its  xbd driver?  A check of my system shows the
max_request_size set to 40960 bytes on FreeBSD-12.2.  I remember a similar mismatch problem
exists between NetBSD-current and NetBSD-9, the former having a maximum transfer length of 64k
and the latter having a maximum transfer length of 32k, leading to all sorts  of corruption
issues.  FreeBSD exports the relevant values to sysctl read-only variables, so it's easy to
check what they are on your system.  The source code for the block front end driver in FreeBSD
suggests these values are negotiated between the bak and front end drivers, but I don't know
how that negotiation works.  Based on the previously mentioned NetBSD bug between NetBSD-9 and
NetBSD-current, however, I'd venture to say the NetBSD drivers don't engage in this negotiation
process, what ever it is.  I'm guessing the other difference between your systems that work and
those that don't is that you're now running the FreeBSD guests in pvh mode instead of hvm mode,
since NetBSD didn't support guests in pvh mode until very recently.  By switching to pvh mode,
you've now introduced all these issues with the xbd driver, which weren't there before because
qemu was sitting between the two hosts pretending to be a disk controller/disk.

Hope that helps, a little.
-Brian