A brief tour of i86xen

In this post, I'm going to give a quick walk through the major changes we've made so far in doing our port of Solaris to the Xen "platform". As we've only supplied a tarball of the source tree so far, I can't hyperlink to the relevant bits - sorry about that. As our code is still under heavy development, you can expect some of this code organisation to change significantly; nonetheless I thought this might be useful for those interested in peeking into the internals of what we've done so far.

As you might expect, the vast majority of the changes we've made reside in the kernel. To support booting Solaris under Xen (both domU and dom0, though as we've said the latter is still in the very early stages of development), we've introduced a new platform based on i86pc called i86xen. Wherever possible, we've tried to share common code by using i86pc's sources. There's still some cleanup we can do in this area.

Within usr/src/uts/i86xen, there are a number of Xen-specific source files:

Contains the PSM ("Platform-Specific Module") module for Xen. This mirrors the PSM provided by i86pc, but deals with the hypervisor-provided features such as the clock timer and the events system.
This contains the virtual root nexus driver "xendev". All of the virtual frontend drivers are connected to this.
The virtual block driver. It's currently non-functional with the version of Xen we're working with; we're working hard on getting it functional.
The guts of the kernel/hypervisor code. Amongst other things, it provides interfaces for dealing with events in evtchn.c and hypervisor_machdep.c (the hypervisor version of virtual interrupts, which hook into Solaris's standard interrupt system), the grant table in gnttab.c (used for providing access/transfer of pages between frontend and backend, suspend/resume in xen_machdep.c, and support routines for the debugger and the MMU code (mach_kdi.c and xen_mmu.c respectively).

As mentioned we use the i86pc code where possible, occasionally using #ifdefs where minor differences are found. In particular we re-use the i86pc HAT (MMU management) code found in i86pc/vm. You can also find code for the new boot method described by Joe Bonasera in i86pc/dboot and i86pc/boot.

A number of drivers that are needed by Xen but aren't i86xen specific live under usr/src/uts/common:

common/io/xenbus_*.c common/io/xenbus/
"xenbus" is a simple transport for configuration data provided by domain0; for example, it provides a node control/shutdown which will notify the domainU that the user has requested the domain to be shutdown (or suspended) from domain0's management tools. This code provides this support.
The virtual console frontend driver.
The virtual net device frontend driver.

As you might expect, the userspace changes we've needed to make so far have been reasonably minimal. Despite supporting the new i86xen platform definition, the only significant changes have been to usr/src/cmd/mdb/, where we've added some changes to better support debugging of the Xen-style x86 MMU.