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We found Vista polls ACPI timer very frequently (about 15 times averagely) when handling timer (RTC @ 64HZ) interrupt routine. Though the exact reason is known, it should be related to system time adjustment. When it's in Qemu, the overhead is big. After moving, Vista's idle overhead decreases dramatically from ~10% to 0.9%.=20 Another benefit is that Vista can only pass Performance rating with this patch. The root cause is that ACPI timer in Qemu isn't synchronous with other platform timer in HV, which results in Vista complains "can't measure TSC frequency". This patch changes vpit.h to vpt.h, for it not only has pit structure in it, but other platform timer's structure. Another change is moving ACPI timer and related address from acpi.h to ioreq.h, which can be shared by HV and ACPI firmware. Signed-off-by: Xiaowei Yang <xiaowei.yang@intel.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