| 1 | Domain firmware support |
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| 2 | ----------------------- |
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| 3 | |
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| 4 | One of the key advantages of full virtualization hardware support (such |
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| 5 | as Intel's VT or AMD's SVM extensions) is the ability to run unmodified |
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| 6 | guest operating systems. However, since most OSes rely on BIOS support |
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| 7 | during their early bringup, we need to provide a surrogate ROMBIOS and |
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| 8 | VGABIOS firmware layer. |
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| 9 | |
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| 10 | What's more, we need to support real-mode which is required by |
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| 11 | the firmware and bootstrap loaders. Real-mode support is especially |
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| 12 | challenging for Intel's VMX (VT) enabled CPUs where there is no real-mode |
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| 13 | support for VMX guest partitions. In this case you either have to do full |
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| 14 | emulation (full real-mode emulator; more complete but potentially slower) |
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| 15 | or partial emulation (use the VM8086 extensions, emulate only those |
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| 16 | instructions that are missing; faster, but potentially incomplete). The |
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| 17 | vmxassist code in this subdirectory uses the later approach because it |
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| 18 | is smaller and faster. |
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| 19 | |
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| 20 | The approach is relatively straight forward. Vmxloader contains three |
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| 21 | payloads (rombios, vgabios and vmxassist) and it is bootstrapped as any |
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| 22 | other 32-bit OS. Vmxloader copies its payloads to the addresses below |
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| 23 | and transfers control to vmxassist. |
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| 24 | |
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| 25 | vgabios VGABIOS (standard and Cirrus). |
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| 26 | Resides at C000:0000. |
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| 27 | |
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| 28 | vmxassist VMXAssist VM86 realmode emulator for VMX. |
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| 29 | Resides at D000:0000. |
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| 30 | |
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| 31 | rombios ROMBIOS code. Derived from Bochs. |
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| 32 | Resides at F000:0000 |
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| 33 | |
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| 34 | Vmxassist first sets up it own world (GDT, IDT, TR, etc), enables |
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| 35 | VM8086 and then transfers control to F000:FFF0 and executes 16-bit |
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| 36 | code. Unsupported instructions cause a general protection failure at |
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| 37 | which point vmxassist kicks in and emulates the offending instruction. |
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| 38 | Whever the emulated code transitions to 32-bit protected mode, vmxassist |
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| 39 | will go away. Whenever 32-bit protected code transitions to real-mode, |
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| 40 | Xen/VMX will detect this and transfer control to vmxassist. |
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| 41 | |
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| 42 | Most of the vmxassist complexity comes from properly handling the |
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| 43 | real to protected mode and protected to real mode transitions and |
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| 44 | the proper emulation of the segment registers. Even though the Intel |
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| 45 | manual clearly states that you should immediately perform a jmp far |
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| 46 | after a mode transition, many operating systems execute additional |
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| 47 | instructions and some even refer to segment selectors and pop data |
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| 48 | from the stack. Vmxassist contains a number of work arounds for these |
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| 49 | OSes. |
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| 50 | |
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| 51 | |
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| 52 | Acknowledgements |
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| 53 | ---------------- |
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| 54 | |
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| 55 | The rombios was taken (largely unmodified) from Bochs, which was written |
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| 56 | by Kevin Lawton. The VGABIOS was written by Christophe Bothamy. Arun Sharma, |
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| 57 | Asit Mallick and Nitin Kamble (Intel) provided the E820 patches and lots |
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| 58 | of useful feedback. |
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| 59 | |
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| 60 | |
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| 61 | Contact |
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| 62 | ------- |
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| 63 | |
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| 64 | Leendert van Doorn |
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| 65 | IBM T.J. Watson Research Center |
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| 66 | 19 Skyline Drive |
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| 67 | Hawthorne, NY 10532 |
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| 68 | leendert@watson.ibm.com |
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| 69 | |
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| 70 | |
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| 71 | Tested Operating Systems |
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| 72 | ------------------------ |
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| 73 | |
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| 74 | Since vmxassist uses partial emulation, it may always miss opcodes |
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| 75 | that are required by a particular OS. The table below lists the OSes |
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| 76 | I have tried. The Install column indicates a full CD/DVD install into |
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| 77 | a VMX partition. The Disk column indicates booting from prefabricated |
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| 78 | disk image. |
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| 79 | |
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| 80 | Operating System Install Disk |
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| 81 | ------------------------------------------------------------ |
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| 82 | RedHat Enterprise Linux (RHEL3_U5) Yes Yes |
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| 83 | Fedora Code (FC3) (-) Yes |
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| 84 | FreeBSD 5.3 (-) Yes |
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| 85 | MS-DOS 5.0 (-) Yes |
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| 86 | |
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| 87 | (-) not tried yet |
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| 88 | |
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