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Commit 1f673135 authored by bellard's avatar bellard
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doc update 


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@705 c046a42c-6fe2-441c-8c8c-71466251a162
parent aa455485
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Makefile
View file @ 1f673135
......@@ -11,7 +11,7 @@ ifndef CONFIG_WIN32
TOOLS=qemu-mkcow
endif
all: dyngen$(EXESUF) $(TOOLS) qemu-doc.html qemu.1
all: dyngen$(EXESUF) $(TOOLS) qemu-doc.html qemu-tech.html qemu.1
for d in $(TARGET_DIRS); do \
make -C $$d $@ || exit 1 ; \
done
......@@ -61,7 +61,7 @@ TAGS:
etags *.[ch] tests/*.[ch]
# documentation
qemu-doc.html: qemu-doc.texi
%.html: %.texi
texi2html -monolithic -number $<
qemu.1: qemu-doc.texi
......
TODO
View file @ 1f673135
......@@ -2,7 +2,6 @@ short term:
----------
- handle fast timers + add explicit clocks
- OS/2 install bug
- win 95 install bug
- handle Self Modifying Code even if modifying current TB (BE OS 5 install)
- physical memory cache (reduce qemu-fast address space size to about 32 MB)
- better code fetch
......
linux-2.6-qemu-fast.patch 0 → 100644
View file @ 1f673135
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/arch/i386/Kconfig .32324-linux-2.6.0.updated/arch/i386/Kconfig
--- .32324-linux-2.6.0/arch/i386/Kconfig 2003-10-09 18:02:48.000000000 +1000
+++ .32324-linux-2.6.0.updated/arch/i386/Kconfig 2003-12-26 16:46:49.000000000 +1100
@@ -307,6 +307,14 @@ config X86_GENERIC
when it has moderate overhead. This is intended for generic
distributions kernels.
+config QEMU
+ bool "Kernel to run under QEMU"
+ depends on EXPERIMENTAL
+ help
+ Select this if you want to boot the kernel inside qemu-fast,
+ the non-mmu version of the x86 emulator. See
+ <http://fabrice.bellard.free.fr/qemu/>. Say N.
+
#
# Define implied options from the CPU selection here
#
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/arch/i386/kernel/Makefile .32324-linux-2.6.0.updated/arch/i386/kernel/Makefile
--- .32324-linux-2.6.0/arch/i386/kernel/Makefile 2003-09-29 10:25:15.000000000 +1000
+++ .32324-linux-2.6.0.updated/arch/i386/kernel/Makefile 2003-12-26 16:46:49.000000000 +1100
@@ -46,12 +46,14 @@ quiet_cmd_syscall = SYSCALL $@
cmd_syscall = $(CC) -nostdlib $(SYSCFLAGS_$(@F)) \
-Wl,-T,$(filter-out FORCE,$^) -o $@
+export AFLAGS_vsyscall.lds.o += -P -C -U$(ARCH)
+
vsyscall-flags = -shared -s -Wl,-soname=linux-gate.so.1
SYSCFLAGS_vsyscall-sysenter.so = $(vsyscall-flags)
SYSCFLAGS_vsyscall-int80.so = $(vsyscall-flags)
$(obj)/vsyscall-int80.so $(obj)/vsyscall-sysenter.so: \
-$(obj)/vsyscall-%.so: $(src)/vsyscall.lds $(obj)/vsyscall-%.o FORCE
+$(obj)/vsyscall-%.so: $(src)/vsyscall.lds.s $(obj)/vsyscall-%.o FORCE
$(call if_changed,syscall)
# We also create a special relocatable object that should mirror the symbol
@@ -62,5 +64,5 @@ $(obj)/built-in.o: $(obj)/vsyscall-syms.
$(obj)/built-in.o: ld_flags += -R $(obj)/vsyscall-syms.o
SYSCFLAGS_vsyscall-syms.o = -r
-$(obj)/vsyscall-syms.o: $(src)/vsyscall.lds $(obj)/vsyscall-sysenter.o FORCE
+$(obj)/vsyscall-syms.o: $(src)/vsyscall.lds.s $(obj)/vsyscall-sysenter.o FORCE
$(call if_changed,syscall)
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/arch/i386/kernel/vmlinux.lds.S .32324-linux-2.6.0.updated/arch/i386/kernel/vmlinux.lds.S
--- .32324-linux-2.6.0/arch/i386/kernel/vmlinux.lds.S 2003-09-22 10:27:28.000000000 +1000
+++ .32324-linux-2.6.0.updated/arch/i386/kernel/vmlinux.lds.S 2003-12-26 16:46:49.000000000 +1100
@@ -3,6 +3,7 @@
*/
#include <asm-generic/vmlinux.lds.h>
+#include <asm/page.h>
OUTPUT_FORMAT("elf32-i386", "elf32-i386", "elf32-i386")
OUTPUT_ARCH(i386)
@@ -10,7 +11,7 @@ ENTRY(startup_32)
jiffies = jiffies_64;
SECTIONS
{
- . = 0xC0000000 + 0x100000;
+ . = __PAGE_OFFSET + 0x100000;
/* read-only */
_text = .; /* Text and read-only data */
.text : {
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/arch/i386/kernel/vsyscall.lds .32324-linux-2.6.0.updated/arch/i386/kernel/vsyscall.lds
--- .32324-linux-2.6.0/arch/i386/kernel/vsyscall.lds 2003-09-22 10:07:26.000000000 +1000
+++ .32324-linux-2.6.0.updated/arch/i386/kernel/vsyscall.lds 1970-01-01 10:00:00.000000000 +1000
@@ -1,67 +0,0 @@
-/*
- * Linker script for vsyscall DSO. The vsyscall page is an ELF shared
- * object prelinked to its virtual address, and with only one read-only
- * segment (that fits in one page). This script controls its layout.
- */
-
-/* This must match <asm/fixmap.h>. */
-VSYSCALL_BASE = 0xffffe000;
-
-SECTIONS
-{
- . = VSYSCALL_BASE + SIZEOF_HEADERS;
-
- .hash : { *(.hash) } :text
- .dynsym : { *(.dynsym) }
- .dynstr : { *(.dynstr) }
- .gnu.version : { *(.gnu.version) }
- .gnu.version_d : { *(.gnu.version_d) }
- .gnu.version_r : { *(.gnu.version_r) }
-
- /* This linker script is used both with -r and with -shared.
- For the layouts to match, we need to skip more than enough
- space for the dynamic symbol table et al. If this amount
- is insufficient, ld -shared will barf. Just increase it here. */
- . = VSYSCALL_BASE + 0x400;
-
- .text : { *(.text) } :text =0x90909090
-
- .eh_frame_hdr : { *(.eh_frame_hdr) } :text :eh_frame_hdr
- .eh_frame : { KEEP (*(.eh_frame)) } :text
- .dynamic : { *(.dynamic) } :text :dynamic
- .useless : {
- *(.got.plt) *(.got)
- *(.data .data.* .gnu.linkonce.d.*)
- *(.dynbss)
- *(.bss .bss.* .gnu.linkonce.b.*)
- } :text
-}
-
-/*
- * We must supply the ELF program headers explicitly to get just one
- * PT_LOAD segment, and set the flags explicitly to make segments read-only.
- */
-PHDRS
-{
- text PT_LOAD FILEHDR PHDRS FLAGS(5); /* PF_R|PF_X */
- dynamic PT_DYNAMIC FLAGS(4); /* PF_R */
- eh_frame_hdr 0x6474e550; /* PT_GNU_EH_FRAME, but ld doesn't match the name */
-}
-
-/*
- * This controls what symbols we export from the DSO.
- */
-VERSION
-{
- LINUX_2.5 {
- global:
- __kernel_vsyscall;
- __kernel_sigreturn;
- __kernel_rt_sigreturn;
-
- local: *;
- };
-}
-
-/* The ELF entry point can be used to set the AT_SYSINFO value. */
-ENTRY(__kernel_vsyscall);
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/arch/i386/kernel/vsyscall.lds.S .32324-linux-2.6.0.updated/arch/i386/kernel/vsyscall.lds.S
--- .32324-linux-2.6.0/arch/i386/kernel/vsyscall.lds.S 1970-01-01 10:00:00.000000000 +1000
+++ .32324-linux-2.6.0.updated/arch/i386/kernel/vsyscall.lds.S 2003-12-26 16:46:49.000000000 +1100
@@ -0,0 +1,67 @@
+/*
+ * Linker script for vsyscall DSO. The vsyscall page is an ELF shared
+ * object prelinked to its virtual address, and with only one read-only
+ * segment (that fits in one page). This script controls its layout.
+ */
+#include <asm/fixmap.h>
+
+VSYSCALL_BASE = __FIXADDR_TOP - 0x1000;
+
+SECTIONS
+{
+ . = VSYSCALL_BASE + SIZEOF_HEADERS;
+
+ .hash : { *(.hash) } :text
+ .dynsym : { *(.dynsym) }
+ .dynstr : { *(.dynstr) }
+ .gnu.version : { *(.gnu.version) }
+ .gnu.version_d : { *(.gnu.version_d) }
+ .gnu.version_r : { *(.gnu.version_r) }
+
+ /* This linker script is used both with -r and with -shared.
+ For the layouts to match, we need to skip more than enough
+ space for the dynamic symbol table et al. If this amount
+ is insufficient, ld -shared will barf. Just increase it here. */
+ . = VSYSCALL_BASE + 0x400;
+
+ .text : { *(.text) } :text =0x90909090
+
+ .eh_frame_hdr : { *(.eh_frame_hdr) } :text :eh_frame_hdr
+ .eh_frame : { KEEP (*(.eh_frame)) } :text
+ .dynamic : { *(.dynamic) } :text :dynamic
+ .useless : {
+ *(.got.plt) *(.got)
+ *(.data .data.* .gnu.linkonce.d.*)
+ *(.dynbss)
+ *(.bss .bss.* .gnu.linkonce.b.*)
+ } :text
+}
+
+/*
+ * We must supply the ELF program headers explicitly to get just one
+ * PT_LOAD segment, and set the flags explicitly to make segments read-only.
+ */
+PHDRS
+{
+ text PT_LOAD FILEHDR PHDRS FLAGS(5); /* PF_R|PF_X */
+ dynamic PT_DYNAMIC FLAGS(4); /* PF_R */
+ eh_frame_hdr 0x6474e550; /* PT_GNU_EH_FRAME, but ld doesn't match the name */
+}
+
+/*
+ * This controls what symbols we export from the DSO.
+ */
+VERSION
+{
+ LINUX_2.5 {
+ global:
+ __kernel_vsyscall;
+ __kernel_sigreturn;
+ __kernel_rt_sigreturn;
+
+ local: *;
+ };
+}
+
+/* The ELF entry point can be used to set the AT_SYSINFO value. */
+ENTRY(__kernel_vsyscall);
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/include/asm-i386/fixmap.h .32324-linux-2.6.0.updated/include/asm-i386/fixmap.h
--- .32324-linux-2.6.0/include/asm-i386/fixmap.h 2003-09-22 10:09:12.000000000 +1000
+++ .32324-linux-2.6.0.updated/include/asm-i386/fixmap.h 2003-12-26 16:46:49.000000000 +1100
@@ -14,6 +14,19 @@
#define _ASM_FIXMAP_H
#include <linux/config.h>
+
+/* used by vmalloc.c, vsyscall.lds.S.
+ *
+ * Leave one empty page between vmalloc'ed areas and
+ * the start of the fixmap.
+ */
+#ifdef CONFIG_QEMU
+#define __FIXADDR_TOP 0xa7fff000
+#else
+#define __FIXADDR_TOP 0xfffff000
+#endif
+
+#ifndef __ASSEMBLY__
#include <linux/kernel.h>
#include <asm/acpi.h>
#include <asm/apicdef.h>
@@ -94,13 +107,8 @@ extern void __set_fixmap (enum fixed_add
#define clear_fixmap(idx) \
__set_fixmap(idx, 0, __pgprot(0))
-/*
- * used by vmalloc.c.
- *
- * Leave one empty page between vmalloc'ed areas and
- * the start of the fixmap.
- */
-#define FIXADDR_TOP (0xfffff000UL)
+#define FIXADDR_TOP ((unsigned long)__FIXADDR_TOP)
+
#define __FIXADDR_SIZE (__end_of_permanent_fixed_addresses << PAGE_SHIFT)
#define FIXADDR_START (FIXADDR_TOP - __FIXADDR_SIZE)
@@ -145,4 +153,5 @@ static inline unsigned long virt_to_fix(
return __virt_to_fix(vaddr);
}
+#endif /* !__ASSEMBLY__ */
#endif
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/include/asm-i386/page.h .32324-linux-2.6.0.updated/include/asm-i386/page.h
--- .32324-linux-2.6.0/include/asm-i386/page.h 2003-09-22 10:06:42.000000000 +1000
+++ .32324-linux-2.6.0.updated/include/asm-i386/page.h 2003-12-26 16:46:49.000000000 +1100
@@ -10,10 +10,10 @@
#define LARGE_PAGE_SIZE (1UL << PMD_SHIFT)
#ifdef __KERNEL__
-#ifndef __ASSEMBLY__
-
#include <linux/config.h>
+#ifndef __ASSEMBLY__
+
#ifdef CONFIG_X86_USE_3DNOW
#include <asm/mmx.h>
@@ -115,12 +115,19 @@ static __inline__ int get_order(unsigned
#endif /* __ASSEMBLY__ */
#ifdef __ASSEMBLY__
+#ifdef CONFIG_QEMU
+#define __PAGE_OFFSET (0x90000000)
+#else
#define __PAGE_OFFSET (0xC0000000)
+#endif /* QEMU */
+#else
+#ifdef CONFIG_QEMU
+#define __PAGE_OFFSET (0x90000000UL)
#else
#define __PAGE_OFFSET (0xC0000000UL)
+#endif /* QEMU */
#endif
-
#define PAGE_OFFSET ((unsigned long)__PAGE_OFFSET)
#define VMALLOC_RESERVE ((unsigned long)__VMALLOC_RESERVE)
#define MAXMEM (-__PAGE_OFFSET-__VMALLOC_RESERVE)
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32324-linux-2.6.0/include/asm-i386/param.h .32324-linux-2.6.0.updated/include/asm-i386/param.h
--- .32324-linux-2.6.0/include/asm-i386/param.h 2003-09-21 17:26:06.000000000 +1000
+++ .32324-linux-2.6.0.updated/include/asm-i386/param.h 2003-12-26 16:46:49.000000000 +1100
@@ -2,7 +2,12 @@
#define _ASMi386_PARAM_H
#ifdef __KERNEL__
-# define HZ 1000 /* Internal kernel timer frequency */
+# include <linux/config.h>
+# ifdef CONFIG_QEMU
+# define HZ 100
+# else
+# define HZ 1000 /* Internal kernel timer frequency */
+# endif
# define USER_HZ 100 /* .. some user interfaces are in "ticks" */
# define CLOCKS_PER_SEC (USER_HZ) /* like times() */
#endif
qemu-doc.texi
View file @ 1f673135
This diff is collapsed.
qemu-tech.texi 0 → 100644
View file @ 1f673135
\input texinfo @c -*- texinfo -*-
@iftex
@settitle QEMU Internals
@titlepage
@sp 7
@center @titlefont{QEMU Internals}
@sp 3
@end titlepage
@end iftex
@chapter Introduction
@section Features
QEMU is a FAST! processor emulator using a portable dynamic
translator.
QEMU has two operating modes:
@itemize @minus
@item
Full system emulation. In this mode, QEMU emulates a full system
(usually a PC), including a processor and various peripherials. It can
be used to launch an different Operating System without rebooting the
PC or to debug system code.
@item
User mode emulation (Linux host only). In this mode, QEMU can launch
Linux processes compiled for one CPU on another CPU. It can be used to
launch the Wine Windows API emulator (@url{http://www.winehq.org}) or
to ease cross-compilation and cross-debugging.
@end itemize
As QEMU requires no host kernel driver to run, it is very safe and
easy to use.
QEMU generic features:
@itemize
@item User space only or full system emulation.
@item Using dynamic translation to native code for reasonnable speed.
@item Working on x86 and PowerPC hosts. Being tested on ARM, Sparc32, Alpha and S390.
@item Self-modifying code support.
@item Precise exceptions support.
@item The virtual CPU is a library (@code{libqemu}) which can be used
in other projects.
@end itemize
QEMU user mode emulation features:
@itemize
@item Generic Linux system call converter, including most ioctls.
@item clone() emulation using native CPU clone() to use Linux scheduler for threads.
@item Accurate signal handling by remapping host signals to target signals.
@end itemize
@end itemize
QEMU full system emulation features:
@itemize
@item QEMU can either use a full software MMU for maximum portability or use the host system call mmap() to simulate the target MMU.
@end itemize
@section x86 emulation
QEMU x86 target features:
@itemize
@item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation.
LDT/GDT and IDT are emulated. VM86 mode is also supported to run DOSEMU.
@item Support of host page sizes bigger than 4KB in user mode emulation.
@item QEMU can emulate itself on x86.
@item An extensive Linux x86 CPU test program is included @file{tests/test-i386}.
It can be used to test other x86 virtual CPUs.
@end itemize
Current QEMU limitations:
@itemize
@item No SSE/MMX support (yet).
@item No x86-64 support.
@item IPC syscalls are missing.
@item The x86 segment limits and access rights are not tested at every
memory access (yet). Hopefully, very few OSes seem to rely on that for
normal use.
@item On non x86 host CPUs, @code{double}s are used instead of the non standard
10 byte @code{long double}s of x86 for floating point emulation to get
maximum performances.
@end itemize
@section ARM emulation
@itemize
@item Full ARM 7 user emulation.
@item NWFPE FPU support included in user Linux emulation.
@item Can run most ARM Linux binaries.
@end itemize
@section PowerPC emulation
@itemize
@item Full PowerPC 32 bit emulation, including priviledged instructions,
FPU and MMU.
@item Can run most PowerPC Linux binaries.
@end itemize
@section SPARC emulation
@itemize
@item SPARC V8 user support, except FPU instructions.
@item Can run some SPARC Linux binaries.
@end itemize
@chapter QEMU Internals
@section QEMU compared to other emulators
Like bochs [3], QEMU emulates an x86 CPU. But QEMU is much faster than
bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC
emulation while QEMU can emulate several processors.
Like Valgrind [2], QEMU does user space emulation and dynamic
translation. Valgrind is mainly a memory debugger while QEMU has no
support for it (QEMU could be used to detect out of bound memory
accesses as Valgrind, but it has no support to track uninitialised data
as Valgrind does). The Valgrind dynamic translator generates better code
than QEMU (in particular it does register allocation) but it is closely
tied to an x86 host and target and has no support for precise exceptions
and system emulation.
EM86 [4] is the closest project to user space QEMU (and QEMU still uses
some of its code, in particular the ELF file loader). EM86 was limited
to an alpha host and used a proprietary and slow interpreter (the
interpreter part of the FX!32 Digital Win32 code translator [5]).
TWIN [6] is a Windows API emulator like Wine. It is less accurate than
Wine but includes a protected mode x86 interpreter to launch x86 Windows
executables. Such an approach as greater potential because most of the
Windows API is executed natively but it is far more difficult to develop
because all the data structures and function parameters exchanged
between the API and the x86 code must be converted.
User mode Linux [7] was the only solution before QEMU to launch a
Linux kernel as a process while not needing any host kernel
patches. However, user mode Linux requires heavy kernel patches while
QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is
slower.
The new Plex86 [8] PC virtualizer is done in the same spirit as the
qemu-fast system emulator. It requires a patched Linux kernel to work
(you cannot launch the same kernel on your PC), but the patches are
really small. As it is a PC virtualizer (no emulation is done except
for some priveledged instructions), it has the potential of being
faster than QEMU. The downside is that a complicated (and potentially
unsafe) host kernel patch is needed.
The commercial PC Virtualizers (VMWare [9], VirtualPC [10], TwoOStwo
[11]) are faster than QEMU, but they all need specific, proprietary
and potentially unsafe host drivers. Moreover, they are unable to
provide cycle exact simulation as an emulator can.
@section Portable dynamic translation
QEMU is a dynamic translator. When it first encounters a piece of code,
it converts it to the host instruction set. Usually dynamic translators
are very complicated and highly CPU dependent. QEMU uses some tricks
which make it relatively easily portable and simple while achieving good
performances.
The basic idea is to split every x86 instruction into fewer simpler
instructions. Each simple instruction is implemented by a piece of C
code (see @file{target-i386/op.c}). Then a compile time tool
(@file{dyngen}) takes the corresponding object file (@file{op.o})
to generate a dynamic code generator which concatenates the simple
instructions to build a function (see @file{op.h:dyngen_code()}).
In essence, the process is similar to [1], but more work is done at
compile time.
A key idea to get optimal performances is that constant parameters can
be passed to the simple operations. For that purpose, dummy ELF
relocations are generated with gcc for each constant parameter. Then,
the tool (@file{dyngen}) can locate the relocations and generate the
appriopriate C code to resolve them when building the dynamic code.
That way, QEMU is no more difficult to port than a dynamic linker.
To go even faster, GCC static register variables are used to keep the
state of the virtual CPU.
@section Register allocation
Since QEMU uses fixed simple instructions, no efficient register
allocation can be done. However, because RISC CPUs have a lot of
register, most of the virtual CPU state can be put in registers without
doing complicated register allocation.
@section Condition code optimisations
Good CPU condition codes emulation (@code{EFLAGS} register on x86) is a
critical point to get good performances. QEMU uses lazy condition code
evaluation: instead of computing the condition codes after each x86
instruction, it just stores one operand (called @code{CC_SRC}), the
result (called @code{CC_DST}) and the type of operation (called
@code{CC_OP}).
@code{CC_OP} is almost never explicitely set in the generated code
because it is known at translation time.
In order to increase performances, a backward pass is performed on the
generated simple instructions (see
@code{target-i386/translate.c:optimize_flags()}). When it can be proved that
the condition codes are not needed by the next instructions, no
condition codes are computed at all.
@section CPU state optimisations
The x86 CPU has many internal states which change the way it evaluates
instructions. In order to achieve a good speed, the translation phase
considers that some state information of the virtual x86 CPU cannot
change in it. For example, if the SS, DS and ES segments have a zero
base, then the translator does not even generate an addition for the
segment base.
[The FPU stack pointer register is not handled that way yet].
@section Translation cache
A 2MByte cache holds the most recently used translations. For
simplicity, it is completely flushed when it is full. A translation unit
contains just a single basic block (a block of x86 instructions
terminated by a jump or by a virtual CPU state change which the
translator cannot deduce statically).
@section Direct block chaining
After each translated basic block is executed, QEMU uses the simulated
Program Counter (PC) and other cpu state informations (such as the CS
segment base value) to find the next basic block.
In order to accelerate the most common cases where the new simulated PC
is known, QEMU can patch a basic block so that it jumps directly to the
next one.
The most portable code uses an indirect jump. An indirect jump makes
it easier to make the jump target modification atomic. On some host
architectures (such as x86 or PowerPC), the @code{JUMP} opcode is
directly patched so that the block chaining has no overhead.
@section Self-modifying code and translated code invalidation
Self-modifying code is a special challenge in x86 emulation because no
instruction cache invalidation is signaled by the application when code
is modified.
When translated code is generated for a basic block, the corresponding
host page is write protected if it is not already read-only (with the
system call @code{mprotect()}). Then, if a write access is done to the
page, Linux raises a SEGV signal. QEMU then invalidates all the
translated code in the page and enables write accesses to the page.
Correct translated code invalidation is done efficiently by maintaining
a linked list of every translated block contained in a given page. Other
linked lists are also maintained to undo direct block chaining.
Although the overhead of doing @code{mprotect()} calls is important,
most MSDOS programs can be emulated at reasonnable speed with QEMU and
DOSEMU.
Note that QEMU also invalidates pages of translated code when it detects
that memory mappings are modified with @code{mmap()} or @code{munmap()}.
When using a software MMU, the code invalidation is more efficient: if
a given code page is invalidated too often because of write accesses,
then a bitmap representing all the code inside the page is
built. Every store into that page checks the bitmap to see if the code
really needs to be invalidated. It avoids invalidating the code when
only data is modified in the page.
@section Exception support
longjmp() is used when an exception such as division by zero is
encountered.
The host SIGSEGV and SIGBUS signal handlers are used to get invalid
memory accesses. The exact CPU state can be retrieved because all the
x86 registers are stored in fixed host registers. The simulated program
counter is found by retranslating the corresponding basic block and by
looking where the host program counter was at the exception point.
The virtual CPU cannot retrieve the exact @code{EFLAGS} register because
in some cases it is not computed because of condition code
optimisations. It is not a big concern because the emulated code can