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21b5e9be6a
The bug occurs when the guest is entering protected mode and reaches the far jump to set up the 32-bit segment registers for the virtual VMENTER we're about to perform. vmxassist, in protected_mode(), looks at the segment registers, which might be 16-bit segments or might be 32-bit segment selectors at this stage (depending on whether the OS has reloaded them since entering protected mode); and it tries to load the segments from the GDT. Unconditionally. Even if the segment register still actually contains a real mode 16-bit segment. Whoops. vmxassist already has code to detect 16-bit segments that survived unmodified from a transition into and out of protected mode, and to save and restore those appropriately. It does this using "saved_rm_regs", which get cleared on entry to protected mode, and then set to the old segment value if we fail to set a given 32-bit segment correctly. The fix is to save the 16-bit segments *always*, on entry to protected mode when %CR0(PE) is first set; and to clear the saved 16-bit segment and set the 32-bit variant in oldctx whenever a 32-bit segment descriptor is set during the transition to 32-bit CS. Then, when we finally do the VMENTER, we will set up the VMCS from only the 32-bit segments, clearing the VMCS entries for segments that have not been assigned valid 32-bit segments yet. Tested on various RHEL-5 boot.isos, including ones which worked before and ones which triggered the bug; all now boot correctly. Signed-off-by: Stephen Tweedie <sct@redhat.com>
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http://www.xensource.com/xen/about.html
What is Xen?
============
Xen is a Virtual Machine Monitor (VMM) originally developed by the
Systems Research Group of the University of Cambridge Computer
Laboratory, as part of the UK-EPSRC funded XenoServers project. Xen
is freely-distributable Open Source software, released under the GNU
GPL. Since its initial public release, Xen has grown a large
development community, spearheaded by XenSource Inc, a company created
by the original Xen development team to build enterprise products
around Xen.
The 3.0 release offers excellent performance, hardware support and
enterprise-grade features such as x86_32-PAE, x86_64, SMP guests and
live relocation of VMs. This install tree contains source for a Linux
2.6 guest; ports to Linux 2.4, NetBSD, FreeBSD and Solaris will follow
later (and are already available for previous Xen releases).
This file contains some quick-start instructions to install Xen on
your system. For full documentation, see the Xen User Manual. If this
is a pre-built release then you can find the manual at:
dist/install/usr/share/doc/xen/pdf/user.pdf
If you have a source release, then 'make -C docs' will build the
manual at docs/pdf/user.pdf.
Quick-Start Guide - Pre-Built Binary Release
============================================
[NB. Unless noted otherwise, all the following steps should be
performed with root privileges.]
1. Install the binary distribution onto your filesystem:
# sh ./install.sh
Among other things, this will install Xen and Xen-ready Linux
kernel files in /boot, kernel modules and Python packages in /lib,
and various control tools in standard 'bin' directories.
2. Configure your bootloader to boot Xen and an initial Linux virtual
machine. Note that Xen currently only works with GRUB and pxelinux
derived boot loaders: less common alternatives such as LILO are
*not* supported. You can most likely find your GRUB menu file at
/boot/grub/menu.lst: edit this file to include an entry like the
following:
title Xen 3.0 / XenLinux 2.6
kernel /boot/xen-3.0.gz console=vga
module /boot/vmlinuz-2.6-xen root=<root-dev> ro console=tty0
module /boot/initrd-2.6-xen.img
NB: Not all kernel configs need an initial ram disk (initrd), but
if you do specify one you'll need to use the 'module' grub directive
rather than 'initrd'.
The linux command line takes all the usual options, such as
root=<root-dev> to specify your usual root partition (e.g.,
/dev/hda1).
The Xen command line takes a number of optional arguments described
in the manual. The most common is 'dom0_mem=xxxM' which sets the
amount of memory to allocate for use by your initial virtual
machine (known as domain 0). Note that Xen itself reserves about
32MB memory for internal use, which is not available for allocation
to virtual machines.
3. Reboot your system and select the "Xen 3.0 / XenLinux 2.6" menu
option. After booting Xen, Linux will start and your initialisation
scripts should execute in the usual way.
Quick-Start Guide - Source Release
==================================
First, there are a number of prerequisites for building a Xen source
release. Make sure you have all the following installed, either by
visiting the project webpage or installing a pre-built package
provided by your Linux distributor:
* GCC (preferably v3.2.x or v3.3.x; older versions are unsupported)
* GNU Make
* GNU Binutils
* Development install of zlib (e.g., zlib-dev)
* Development install of Python v2.3 or later (e.g., python-dev)
* Development install of curses (e.g., libncurses-dev)
* bridge-utils package (/sbin/brctl)
* iproute package (/sbin/ip)
* hotplug or udev
[NB. Unless noted otherwise, all the following steps should be
performed with root privileges.]
1. Download and untar the source tarball file. This will be a
file named xen-unstable-src.tgz, or xen-$version-src.tgz.
You can also pull the current version from the SCMS
that is being used (Bitkeeper, scheduled to change shortly).
# tar xzf xen-unstable-src.tgz
Assuming you are using the unstable tree, this will
untar into xen-unstable. The rest of the instructions
use the unstable tree as an example, substitute the
version for unstable.
2. cd to xen-unstable (or whatever you sensibly rename it to).
The Linux, netbsd and freebsd kernel source trees are in
the $os-$version-xen-sparse directories.
On Linux:
3. For the very first build, or if you want to destroy existing
.configs and build trees, perform the following steps:
# make world
# make install
This will create and install onto the local machine. It will build
the xen binary (xen.gz), and a linux kernel and modules that can be
used in both dom0 and an unprivileged guest kernel (vmlinuz-2.6.x-xen),
the tools and the documentation.
You can override the destination for make install by setting DESTDIR
to some value.
The make command line defaults to building the kernel vmlinuz-2.6.x-xen.
You can override this default by specifying KERNELS=kernelname. For
example, you can make two kernels - linux-2.6-xen0
and linux-2.6-xenU - which are smaller builds containing only selected
modules, intended primarily for developers that don't like to wait
for a full -xen kernel to build. The -xenU kernel is particularly small,
as it does not contain any physical device drivers, and hence is
only useful for guest domains.
To make these two kernels, simply specify
KERNELS="linux-2.6-xen0 linux-2.6-xenU"
in the make command line.
If you want to build an x86_32 PAE capable xen and kernel to work
on machines with >= 4GB of memory, use XEN_TARGET_X86_PAE=y on the
make command line.
4. To rebuild an existing tree without modifying the config:
# make dist
This will build and install xen, kernels, tools, and
docs into the local dist/ directory.
You can override the destination for make install by setting DISTDIR
to some value.
make install and make dist differ in that make install does the
right things for your local machine (installing the appropriate
version of hotplug or udev scripts, for example), but make dist
includes all versions of those scripts, so that you can copy the dist
directory to another machine and install from that distribution.
5. To rebuild a kernel with a modified config:
# make linux-2.6-xen-config CONFIGMODE=menuconfig (or xconfig)
# make linux-2.6-xen-build
# make linux-2.6-xen-install
Depending on your config, you may need to use 'mkinitrd' to create
an initial ram disk, just like a native system e.g.
# depmod 2.6.16-xen
# mkinitrd -v -f --with=aacraid --with=sd_mod --with=scsi_mod initrd-2.6.16-xen.img 2.6.16-xen