mirror of
https://github.com/xen-project/xen.git
synced 2026-10-28 18:21:39 +01:00
kaf24@firebug.cl.cam.ac.uk
13e63314c4
The following allows you to run unmodified guest operating systems
under Xen on VMX (VT) enabled processors. The tree lives under
<ROOT>/tools/dfw. Instead of booting a guest kernel, boot vmxloader and
specify the disk image in qemurc.
Leendert
Signed-Off-By: Leendert van Doorn <leendert@watson.ibm.com>
89 lines
3.3 KiB
Plaintext
89 lines
3.3 KiB
Plaintext
Domain FirmWare support
|
|
-----------------------
|
|
|
|
One of the key advantages of full virtualization hardware support (such
|
|
as Intel's VT or AMD's Pacifica) is the ability to run unmodified guest
|
|
operating systems. However, since most OSes rely on BIOS support during
|
|
their early bringup, we need to provide a surrogate ROMBIOS and VGABIOS
|
|
firmware layer.
|
|
|
|
What's more, we need to support real-mode which is required by
|
|
the firmware and bootstrap loaders. Real-mode support is especially
|
|
challenging for Intel's VMX (VT) enabled CPUs where there is no real-mode
|
|
support for VMX guest partitions. In this case you either have to do full
|
|
emulation (full real-mode emulator; more complete but potentially slower)
|
|
or partial emulation (use the VM8086 extensions, emulate only those
|
|
instructions that are missing; faster, but potentially incomplete). The
|
|
vmxassist code in this subdirectory uses the later approach because it
|
|
is smaller and faster.
|
|
|
|
The approach is relatively straight forward. Vmxloader contains three
|
|
payloads (rombios, vgabios and vmxassist) and it is bootstrapped as any
|
|
other 32-bit OS. Vmxloader copies its payloads to the addresses below
|
|
and transfers control to vmxassist.
|
|
|
|
vgabios VGABIOS (standard and Cirrus).
|
|
Resides at C000:0000.
|
|
|
|
vmxassist VMXAssist VM86 realmode emulator for VMX.
|
|
Resides at D000:0000.
|
|
|
|
rombios ROMBIOS code. Derived from Bochs.
|
|
Resides at F000:0000
|
|
|
|
Vmxassist first sets up it own world (GDT, IDT, TR, etc), enables
|
|
VM8086 and then transfers control to F000:FFF0 and executes 16-bit
|
|
code. Unsupported instructions cause a general protection failure at
|
|
which point vmxassist kicks in and emulates the offending instruction.
|
|
Whever the emulated code transitions to 32-bit protected mode, vmxassist
|
|
will go away. Whenever 32-bit protected code transitions to real-mode,
|
|
Xen/VMX will detect this and transfer control to vmxassist.
|
|
|
|
Most of the vmxassist complexity comes from properly handling the
|
|
real to protected mode and protected to real mode transitions and
|
|
the proper emulation of the segment registers. Even though the Intel
|
|
manual clearly states that you should immediately perform a jmp far
|
|
after a mode transition, many operating systems execute additional
|
|
instructions and some even refer to segment selectors and pop data
|
|
from the stack. Vmxassist contains a number of work arounds for these
|
|
OSes.
|
|
|
|
|
|
Acknowledgements
|
|
----------------
|
|
|
|
The rombios was taken (largely unmodified) from Bochs, which was written
|
|
by Kevin Lawton. The VGABIOS was written by Christophe Bothamy. Arun Sharma,
|
|
Asit Mallick and Nitin Kamble (Intel) provided the E820 patches and lots
|
|
of useful feedback.
|
|
|
|
|
|
Contact
|
|
-------
|
|
|
|
Leendert van Doorn
|
|
IBM T.J. Watson Research Center
|
|
19 Skyline Drive
|
|
Hawthorne, NY 10532
|
|
leendert@watson.ibm.com
|
|
|
|
|
|
Tested Operating Systems
|
|
------------------------
|
|
|
|
Since vmxassist uses partial emulation, it may always miss opcodes
|
|
that are required by a particular OS. The table below lists the OSes
|
|
I have tried. The Install column indicates a full CD/DVD install into
|
|
a VMX partition. The Disk column indicates booting from prefabricated
|
|
disk image.
|
|
|
|
Operating System Install Disk
|
|
------------------------------------------------------------
|
|
RedHat Enterprise Linux (RHEL3_U5) Yes Yes
|
|
Fedora Code (FC3) (-) Yes
|
|
FreeBSD 5.3 (-) Yes
|
|
MS-DOS 5.0 (-) Yes
|
|
|
|
(-) not tried yet
|
|
|