view tools/firmware/README @ 16715:c5deb251b9dc

Update version to 3.2.0-rc4
author Keir Fraser <>
date Sat Dec 29 17:57:37 2007 +0000 (2007-12-29)
parents f1b361b05bf3
line source
1 Domain firmware support
2 -----------------------
4 One of the key advantages of full virtualization hardware support (such
5 as Intel's VT or AMD's SVM extensions) is the ability to run unmodified
6 guest operating systems. However, since most OSes rely on BIOS support
7 during their early bringup, we need to provide a surrogate ROMBIOS and
8 VGABIOS firmware layer.
10 What's more, we need to support real-mode which is required by
11 the firmware and bootstrap loaders. Real-mode support is especially
12 challenging for Intel's VMX (VT) enabled CPUs where there is no real-mode
13 support for VMX guest partitions. In this case you either have to do full
14 emulation (full real-mode emulator; more complete but potentially slower)
15 or partial emulation (use the VM8086 extensions, emulate only those
16 instructions that are missing; faster, but potentially incomplete). The
17 vmxassist code in this subdirectory uses the later approach because it
18 is smaller and faster.
20 The approach is relatively straight forward. Vmxloader contains three
21 payloads (rombios, vgabios and vmxassist) and it is bootstrapped as any
22 other 32-bit OS. Vmxloader copies its payloads to the addresses below
23 and transfers control to vmxassist.
25 vgabios VGABIOS (standard and Cirrus).
26 Resides at C000:0000.
28 vmxassist VMXAssist VM86 realmode emulator for VMX.
29 Resides at D000:0000.
31 rombios ROMBIOS code. Derived from Bochs.
32 Resides at F000:0000
34 Vmxassist first sets up it own world (GDT, IDT, TR, etc), enables
35 VM8086 and then transfers control to F000:FFF0 and executes 16-bit
36 code. Unsupported instructions cause a general protection failure at
37 which point vmxassist kicks in and emulates the offending instruction.
38 Whever the emulated code transitions to 32-bit protected mode, vmxassist
39 will go away. Whenever 32-bit protected code transitions to real-mode,
40 Xen/VMX will detect this and transfer control to vmxassist.
42 Most of the vmxassist complexity comes from properly handling the
43 real to protected mode and protected to real mode transitions and
44 the proper emulation of the segment registers. Even though the Intel
45 manual clearly states that you should immediately perform a jmp far
46 after a mode transition, many operating systems execute additional
47 instructions and some even refer to segment selectors and pop data
48 from the stack. Vmxassist contains a number of work arounds for these
49 OSes.
52 Acknowledgements
53 ----------------
55 The rombios was taken (largely unmodified) from Bochs, which was written
56 by Kevin Lawton. The VGABIOS was written by Christophe Bothamy. Arun Sharma,
57 Asit Mallick and Nitin Kamble (Intel) provided the E820 patches and lots
58 of useful feedback.
61 Contact
62 -------
64 Leendert van Doorn
65 IBM T.J. Watson Research Center
66 19 Skyline Drive
67 Hawthorne, NY 10532
71 Tested Operating Systems
72 ------------------------
74 Since vmxassist uses partial emulation, it may always miss opcodes
75 that are required by a particular OS. The table below lists the OSes
76 I have tried. The Install column indicates a full CD/DVD install into
77 a VMX partition. The Disk column indicates booting from prefabricated
78 disk image.
80 Operating System Install Disk
81 ------------------------------------------------------------
82 RedHat Enterprise Linux (RHEL3_U5) Yes Yes
83 Fedora Code (FC3) (-) Yes
84 FreeBSD 5.3 (-) Yes
85 MS-DOS 5.0 (-) Yes
87 (-) not tried yet