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Jeremy Fitzhardinge 5ead97c84f xen: Core Xen implementation 
This patch is a rollup of all the core pieces of the Xen
implementation, including:
 - booting and setup
 - pagetable setup
 - privileged instructions
 - segmentation
 - interrupt flags
 - upcalls
 - multicall batching

BOOTING AND SETUP

The vmlinux image is decorated with ELF notes which tell the Xen
domain builder what the kernel's requirements are; the domain builder
then constructs the address space accordingly and starts the kernel.

Xen has its own entrypoint for the kernel (contained in an ELF note).
The ELF notes are set up by xen-head.S, which is included into head.S.
In principle it could be linked separately, but it seems to provoke
lots of binutils bugs.

Because the domain builder starts the kernel in a fairly sane state
(32-bit protected mode, paging enabled, flat segments set up), there's
not a lot of setup needed before starting the kernel proper.  The main
steps are:
  1. Install the Xen paravirt_ops, which is simply a matter of a
     structure assignment.
  2. Set init_mm to use the Xen-supplied pagetables (analogous to the
     head.S generated pagetables in a native boot).
  3. Reserve address space for Xen, since it takes a chunk at the top
     of the address space for its own use.
  4. Call start_kernel()

PAGETABLE SETUP

Once we hit the main kernel boot sequence, it will end up calling back
via paravirt_ops to set up various pieces of Xen specific state.  One
of the critical things which requires a bit of extra care is the
construction of the initial init_mm pagetable.  Because Xen places
tight constraints on pagetables (an active pagetable must always be
valid, and must always be mapped read-only to the guest domain), we
need to be careful when constructing the new pagetable to keep these
constraints in mind.  It turns out that the easiest way to do this is
use the initial Xen-provided pagetable as a template, and then just
insert new mappings for memory where a mapping doesn't already exist.

This means that during pagetable setup, it uses a special version of
xen_set_pte which ignores any attempt to remap a read-only page as
read-write (since Xen will map its own initial pagetable as RO), but
lets other changes to the ptes happen, so that things like NX are set
properly.

PRIVILEGED INSTRUCTIONS AND SEGMENTATION

When the kernel runs under Xen, it runs in ring 1 rather than ring 0.
This means that it is more privileged than user-mode in ring 3, but it
still can't run privileged instructions directly.  Non-performance
critical instructions are dealt with by taking a privilege exception
and trapping into the hypervisor and emulating the instruction, but
more performance-critical instructions have their own specific
paravirt_ops.  In many cases we can avoid having to do any hypercalls
for these instructions, or the Xen implementation is quite different
from the normal native version.

The privileged instructions fall into the broad classes of:
  Segmentation: setting up the GDT and the GDT entries, LDT,
     TLS and so on.  Xen doesn't allow the GDT to be directly
     modified; all GDT updates are done via hypercalls where the new
     entries can be validated.  This is important because Xen uses
     segment limits to prevent the guest kernel from damaging the
     hypervisor itself.
  Traps and exceptions: Xen uses a special format for trap entrypoints,
     so when the kernel wants to set an IDT entry, it needs to be
     converted to the form Xen expects.  Xen sets int 0x80 up specially
     so that the trap goes straight from userspace into the guest kernel
     without going via the hypervisor.  sysenter isn't supported.
  Kernel stack: The esp0 entry is extracted from the tss and provided to
     Xen.
  TLB operations: the various TLB calls are mapped into corresponding
     Xen hypercalls.
  Control registers: all the control registers are privileged.  The most
     important is cr3, which points to the base of the current pagetable,
     and we handle it specially.

Another instruction we treat specially is CPUID, even though its not
privileged.  We want to control what CPU features are visible to the
rest of the kernel, and so CPUID ends up going into a paravirt_op.
Xen implements this mainly to disable the ACPI and APIC subsystems.

INTERRUPT FLAGS

Xen maintains its own separate flag for masking events, which is
contained within the per-cpu vcpu_info structure.  Because the guest
kernel runs in ring 1 and not 0, the IF flag in EFLAGS is completely
ignored (and must be, because even if a guest domain disables
interrupts for itself, it can't disable them overall).

(A note on terminology: "events" and interrupts are effectively
synonymous.  However, rather than using an "enable flag", Xen uses a
"mask flag", which blocks event delivery when it is non-zero.)

There are paravirt_ops for each of cli/sti/save_fl/restore_fl, which
are implemented to manage the Xen event mask state.  The only thing
worth noting is that when events are unmasked, we need to explicitly
see if there's a pending event and call into the hypervisor to make
sure it gets delivered.

UPCALLS

Xen needs a couple of upcall (or callback) functions to be implemented
by each guest.  One is the event upcalls, which is how events
(interrupts, effectively) are delivered to the guests.  The other is
the failsafe callback, which is used to report errors in either
reloading a segment register, or caused by iret.  These are
implemented in i386/kernel/entry.S so they can jump into the normal
iret_exc path when necessary.

MULTICALL BATCHING

Xen provides a multicall mechanism, which allows multiple hypercalls
to be issued at once in order to mitigate the cost of trapping into
the hypervisor.  This is particularly useful for context switches,
since the 4-5 hypercalls they would normally need (reload cr3, update
TLS, maybe update LDT) can be reduced to one.  This patch implements a
generic batching mechanism for hypercalls, which gets used in many
places in the Xen code.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Cc: Ian Pratt <ian.pratt@xensource.com>
Cc: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
Cc: Adrian Bunk <bunk@stusta.de>
2007-07-18 08:47:42 -07:00

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Makefile
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#
# i386/Makefile
#
# This file is included by the global makefile so that you can add your own
# architecture-specific flags and dependencies. Remember to do have actions
# for "archclean" cleaning up for this architecture.
#
# This file is subject to the terms and conditions of the GNU General Public
# License. See the file "COPYING" in the main directory of this archive
# for more details.
#
# Copyright (C) 1994 by Linus Torvalds
#
# 19990713 Artur Skawina <skawina@geocities.com>
# Added '-march' and '-mpreferred-stack-boundary' support
#
# 20050320 Kianusch Sayah Karadji <kianusch@sk-tech.net>
# Added support for GEODE CPU
HAS_BIARCH := $(call cc-option-yn, -m32)
ifeq ($(HAS_BIARCH),y)
AS := $(AS) --32
LD := $(LD) -m elf_i386
CC := $(CC) -m32
endif
LDFLAGS := -m elf_i386
OBJCOPYFLAGS := -O binary -R .note -R .comment -S
ifdef CONFIG_RELOCATABLE
LDFLAGS_vmlinux := --emit-relocs
endif
CHECKFLAGS += -D__i386__
CFLAGS += -pipe -msoft-float -mregparm=3 -freg-struct-return
# prevent gcc from keeping the stack 16 byte aligned
CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2)
# CPU-specific tuning. Anything which can be shared with UML should go here.
include $(srctree)/arch/i386/Makefile.cpu
# temporary until string.h is fixed
cflags-y += -ffreestanding
# this works around some issues with generating unwind tables in older gccs
# newer gccs do it by default
cflags-y += -maccumulate-outgoing-args
# Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use
# a lot more stack due to the lack of sharing of stacklots:
CFLAGS += $(shell if [ $(call cc-version) -lt 0400 ] ; then echo $(call cc-option,-fno-unit-at-a-time); fi ;)
# do binutils support CFI?
cflags-y += $(call as-instr,.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_CFI=1,)
AFLAGS += $(call as-instr,.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_CFI=1,)
# is .cfi_signal_frame supported too?
cflags-y += $(call as-instr,.cfi_startproc\n.cfi_signal_frame\n.cfi_endproc,-DCONFIG_AS_CFI_SIGNAL_FRAME=1,)
AFLAGS += $(call as-instr,.cfi_startproc\n.cfi_signal_frame\n.cfi_endproc,-DCONFIG_AS_CFI_SIGNAL_FRAME=1,)
CFLAGS += $(cflags-y)
# Default subarch .c files
mcore-y := mach-default
# Voyager subarch support
mflags-$(CONFIG_X86_VOYAGER) := -Iinclude/asm-i386/mach-voyager
mcore-$(CONFIG_X86_VOYAGER) := mach-voyager
# VISWS subarch support
mflags-$(CONFIG_X86_VISWS) := -Iinclude/asm-i386/mach-visws
mcore-$(CONFIG_X86_VISWS) := mach-visws
# NUMAQ subarch support
mflags-$(CONFIG_X86_NUMAQ) := -Iinclude/asm-i386/mach-numaq
mcore-$(CONFIG_X86_NUMAQ) := mach-default
# BIGSMP subarch support
mflags-$(CONFIG_X86_BIGSMP) := -Iinclude/asm-i386/mach-bigsmp
mcore-$(CONFIG_X86_BIGSMP) := mach-default
#Summit subarch support
mflags-$(CONFIG_X86_SUMMIT) := -Iinclude/asm-i386/mach-summit
mcore-$(CONFIG_X86_SUMMIT) := mach-default
# generic subarchitecture
mflags-$(CONFIG_X86_GENERICARCH) := -Iinclude/asm-i386/mach-generic
mcore-$(CONFIG_X86_GENERICARCH) := mach-default
core-$(CONFIG_X86_GENERICARCH) += arch/i386/mach-generic/
# ES7000 subarch support
mflags-$(CONFIG_X86_ES7000) := -Iinclude/asm-i386/mach-es7000
mcore-$(CONFIG_X86_ES7000) := mach-default
core-$(CONFIG_X86_ES7000) := arch/i386/mach-es7000/
# Xen paravirtualization support
core-$(CONFIG_XEN) += arch/i386/xen/
# default subarch .h files
mflags-y += -Iinclude/asm-i386/mach-default
head-y := arch/i386/kernel/head.o arch/i386/kernel/init_task.o
libs-y += arch/i386/lib/
core-y += arch/i386/kernel/ \
arch/i386/mm/ \
arch/i386/$(mcore-y)/ \
arch/i386/crypto/
drivers-$(CONFIG_MATH_EMULATION) += arch/i386/math-emu/
drivers-$(CONFIG_PCI) += arch/i386/pci/
# must be linked after kernel/
drivers-$(CONFIG_OPROFILE) += arch/i386/oprofile/
drivers-$(CONFIG_PM) += arch/i386/power/
drivers-$(CONFIG_FB) += arch/i386/video/
CFLAGS += $(mflags-y)
AFLAGS += $(mflags-y)
boot := arch/i386/boot
PHONY += zImage bzImage compressed zlilo bzlilo \
zdisk bzdisk fdimage fdimage144 fdimage288 isoimage install
all: bzImage
# KBUILD_IMAGE specify target image being built
KBUILD_IMAGE := $(boot)/bzImage
zImage zlilo zdisk: KBUILD_IMAGE := arch/i386/boot/zImage
zImage bzImage: vmlinux
$(Q)$(MAKE) $(build)=$(boot) $(KBUILD_IMAGE)
compressed: zImage
zlilo bzlilo: vmlinux
$(Q)$(MAKE) $(build)=$(boot) BOOTIMAGE=$(KBUILD_IMAGE) zlilo
zdisk bzdisk: vmlinux
$(Q)$(MAKE) $(build)=$(boot) BOOTIMAGE=$(KBUILD_IMAGE) zdisk
fdimage fdimage144 fdimage288 isoimage: vmlinux
$(Q)$(MAKE) $(build)=$(boot) BOOTIMAGE=$(KBUILD_IMAGE) $@
install:
$(Q)$(MAKE) $(build)=$(boot) BOOTIMAGE=$(KBUILD_IMAGE) install
archclean:
$(Q)$(MAKE) $(clean)=arch/i386/boot
define archhelp
echo '* bzImage - Compressed kernel image (arch/$(ARCH)/boot/bzImage)'
echo ' install - Install kernel using'
echo ' (your) ~/bin/installkernel or'
echo ' (distribution) /sbin/installkernel or'
echo ' install to $$(INSTALL_PATH) and run lilo'
echo ' bzdisk - Create a boot floppy in /dev/fd0'
echo ' fdimage - Create a boot floppy image'
echo ' isoimage - Create a boot CD-ROM image'
endef
CLEAN_FILES += arch/$(ARCH)/boot/fdimage \
arch/$(ARCH)/boot/image.iso \
arch/$(ARCH)/boot/mtools.conf
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