mirror of
https://github.com/xen-project/xen.git
synced 2026-10-28 10:11:27 +01:00
kaf24@localhost.localdomain
dd59dfdfb1
compiled with gcc and linked into the hvmloader as a byte-array. Hvmloader allocates memory (rounded up to next 64kb) from the e820 table below 4GB, copies and relocates the 32bit code in the allocated area and copies a jumptable (located in a section '.biosjumptable') pointing to the 'exported' functions into the Bochs BIOS's memory area to link the two code sections. The memory area has been reserved and can be identified with the signature '___JMPT'. In the Bochs BIOS only stub functions are provided. These load the index of a particular function in the jump table, switch to protected mode and call the function in the high memory area. The stack is prepared such that functions compiled by gcc can just pick the parameters from the stack as usual - this means that the 16bit real-mode return address is taken off the stack. The stub functions should have the same signature as those in 32bit space. For ABI compatibility reasons parameters inside the Bochs BIOS stubs should all be 32bit wide. This patch includes a test function that calls three gcc-compiled functions in the high memory area and displays their success status. Simple tests are done doing multiplication and addition of 32-bit numbers and reading and modification of a static variable. These functions test the interface and the relocation code. The test code is removed in patch part 3. Signed-off-by: Stefan Berger <stefanb@us.ibm.com>
Domain firmware support ----------------------- One of the key advantages of full virtualization hardware support (such as Intel's VT or AMD's SVM extensions) 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