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* fix some minor syntax issues in docs/specs/index.rst
  * build and install the 'specs' manual, since it now has some content
  * delete the "QEMU compared to other emulators" section of the docs
  * Convert "translator internals" docs to RST, move to devel manual
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Merge remote-tracking branch 'remotes/pmaydell/tags/pull-docs-20190617' into staging

docs infrastructure queue:
 * fix some minor syntax issues in docs/specs/index.rst
 * build and install the 'specs' manual, since it now has some content
 * delete the "QEMU compared to other emulators" section of the docs
 * Convert "translator internals" docs to RST, move to devel manual

# gpg: Signature made Mon 17 Jun 2019 15:56:07 BST
# gpg:                using RSA key E1A5C593CD419DE28E8315CF3C2525ED14360CDE
# gpg:                issuer "peter.maydell@linaro.org"
# gpg: Good signature from "Peter Maydell <peter.maydell@linaro.org>" [ultimate]
# gpg:                 aka "Peter Maydell <pmaydell@gmail.com>" [ultimate]
# gpg:                 aka "Peter Maydell <pmaydell@chiark.greenend.org.uk>" [ultimate]
# Primary key fingerprint: E1A5 C593 CD41 9DE2 8E83  15CF 3C25 25ED 1436 0CDE

* remotes/pmaydell/tags/pull-docs-20190617:
  docs: Build and install specs manual
  docs/specs/index.rst: Fix minor syntax issues
  qemu-tech.texi: Remove "QEMU compared to other emulators" section
  Convert "translator internals" docs to RST, move to devel manual

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Peter Maydell 2019-06-17 16:41:25 +01:00
commit 076243ffe6
6 changed files with 139 additions and 215 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Makefile
View File

@ -731,6 +731,7 @@ distclean: clean
rm -rf .doctrees
$(call clean-manual,devel)
$(call clean-manual,interop)
$(call clean-manual,specs)
for d in $(TARGET_DIRS); do \
rm -rf $$d || exit 1 ; \
done
@ -781,6 +782,7 @@ endef
.PHONY: install-sphinxdocs
install-sphinxdocs: sphinxdocs
$(call install-manual,interop)
$(call install-manual,specs)
install-doc: $(DOCS) install-sphinxdocs
$(INSTALL_DIR) "$(DESTDIR)$(qemu_docdir)"
@ -962,7 +964,7 @@ docs/version.texi: $(SRC_PATH)/VERSION config-host.mak
# and handles "don't rebuild things unless necessary" itself.
# The '.doctrees' files are cached information to speed this up.
.PHONY: sphinxdocs
sphinxdocs: $(MANUAL_BUILDDIR)/devel/index.html $(MANUAL_BUILDDIR)/interop/index.html
sphinxdocs: $(MANUAL_BUILDDIR)/devel/index.html $(MANUAL_BUILDDIR)/interop/index.html $(MANUAL_BUILDDIR)/specs/index.html
# Canned command to build a single manual
build-manual = $(call quiet-command,sphinx-build $(if $(V),,-q) -W -n -b html -D version=$(VERSION) -D release="$(FULL_VERSION)" -d .doctrees/$1 $(SRC_PATH)/docs/$1 $(MANUAL_BUILDDIR)/$1 ,"SPHINX","$(MANUAL_BUILDDIR)/$1")
@ -975,6 +977,9 @@ $(MANUAL_BUILDDIR)/devel/index.html: $(call manual-deps,devel)
$(MANUAL_BUILDDIR)/interop/index.html: $(call manual-deps,interop)
$(call build-manual,interop)
$(MANUAL_BUILDDIR)/specs/index.html: $(call manual-deps,specs)
$(call build-manual,specs)
qemu-options.texi: $(SRC_PATH)/qemu-options.hx $(SRC_PATH)/scripts/hxtool
$(call quiet-command,sh $(SRC_PATH)/scripts/hxtool -t < $< > $@,"GEN","$@")

1
docs/devel/index.rst
View File

@ -21,3 +21,4 @@ Contents:
testing
decodetree
secure-coding-practices
tcg

111
docs/devel/tcg.rst Normal file
View File

@ -0,0 +1,111 @@
====================
Translator Internals
====================
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.
QEMU's dynamic translation backend is called TCG, for "Tiny Code
Generator". For more information, please take a look at ``tcg/README``.
Some notable features of QEMU's dynamic translator are:
CPU state optimisations
-----------------------
The target CPUs have 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
CPU cannot change in it. The state is recorded in the Translation
Block (TB). If the state changes (e.g. privilege level), a new TB will
be generated and the previous TB won't be used anymore until the state
matches the state recorded in the previous TB. The same idea can be applied
to other aspects of the CPU state. For example, on x86, if the SS,
DS and ES segments have a zero base, then the translator does not even
generate an addition for the segment base.
Direct block chaining
---------------------
After each translated basic block is executed, QEMU uses the simulated
Program Counter (PC) and other cpu state information (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 ``JUMP`` opcode is
directly patched so that the block chaining has no overhead.
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.
User-mode emulation marks a host page as write-protected (if it is
not already read-only) every time translated code is generated for a
basic block. 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. For system emulation, write
protection is achieved through the software MMU.
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.
On RISC targets, correctly written software uses memory barriers and
cache flushes, so some of the protection above would not be
necessary. However, QEMU still requires that the generated code always
matches the target instructions in memory in order to handle
exceptions correctly.
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. QEMU keeps a map from host program counter to
target program counter, and looks up where the exception happened
based on the host program counter at the exception point.
On some targets, some bits of the virtual CPU's state are not flushed to the
memory until the end of the translation block. This is done for internal
emulation state that is rarely accessed directly by the program and/or changes
very often throughout the execution of a translation block---this includes
condition codes on x86, delay slots on SPARC, conditional execution on
ARM, and so on. This state is stored for each target instruction, and
looked up on exceptions.
MMU emulation
-------------
For system emulation QEMU uses a software MMU. In that mode, the MMU
virtual to physical address translation is done at every memory
access.
QEMU uses an address translation cache (TLB) to speed up the translation.
In order to avoid flushing the translated code each time the MMU
mappings change, all caches in QEMU are physically indexed. This
means that each basic block is indexed with its physical address.
In order to avoid invalidating the basic block chain when MMU mappings
change, chaining is only performed when the destination of the jump
shares a page with the basic block that is performing the jump.
The MMU can also distinguish RAM and ROM memory areas from MMIO memory
areas. Access is faster for RAM and ROM because the translation cache also
hosts the offset between guest address and host memory. Accessing MMIO
memory areas instead calls out to C code for device emulation.
Finally, the MMU helps tracking dirty pages and pages pointed to by
translation blocks.

16
docs/specs/conf.py Normal file
View File

@ -0,0 +1,16 @@
# -*- coding: utf-8 -*-
#
# QEMU documentation build configuration file for the 'specs' manual.
#
# This includes the top level conf file and then makes any necessary tweaks.
import sys
import os
qemu_docdir = os.path.abspath("..")
parent_config = os.path.join(qemu_docdir, "conf.py")
exec(compile(open(parent_config, "rb").read(), parent_config, 'exec'))
# This slightly misuses the 'description', but is the best way to get
# the manual title to appear in the sidebar.
html_theme_options['description'] = \
u'System Emulation Guest Hardware Specifications'

9
docs/specs/index.rst
View File

@ -1,8 +1,8 @@
. This is the top level page for the 'specs' manual
.. This is the top level page for the 'specs' manual
QEMU full-system emulation guest hardware specifications
========================================================
QEMU System Emulation Guest Hardware Specifications
===================================================
Contents:
@ -10,4 +10,5 @@ Contents:
.. toctree::
:maxdepth: 2
xive
ppc-xive
ppc-spapr-xive

210
qemu-tech.texi
View File

@ -161,160 +161,6 @@ may be created from overlay with minimal amount of hand-written code.
@end itemize
@node Translator Internals
@section Translator Internals
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.
QEMU's dynamic translation backend is called TCG, for "Tiny Code
Generator". For more information, please take a look at @code{tcg/README}.
Some notable features of QEMU's dynamic translator are:
@table @strong
@item CPU state optimisations:
The target CPUs have 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
CPU cannot change in it. The state is recorded in the Translation
Block (TB). If the state changes (e.g. privilege level), a new TB will
be generated and the previous TB won't be used anymore until the state
matches the state recorded in the previous TB. The same idea can be applied
to other aspects of the CPU state. For example, on x86, if the SS,
DS and ES segments have a zero base, then the translator does not even
generate an addition for the segment base.
@item Direct block chaining:
After each translated basic block is executed, QEMU uses the simulated
Program Counter (PC) and other cpu state information (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.
@item 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.
User-mode emulation marks a host page as write-protected (if it is
not already read-only) every time translated code is generated for a
basic block. 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. For system emulation, write
protection is achieved through the software MMU.
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.
On RISC targets, correctly written software uses memory barriers and
cache flushes, so some of the protection above would not be
necessary. However, QEMU still requires that the generated code always
matches the target instructions in memory in order to handle
exceptions correctly.
@item 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. QEMU keeps a map from host program counter to
target program counter, and looks up where the exception happened
based on the host program counter at the exception point.
On some targets, some bits of the virtual CPU's state are not flushed to the
memory until the end of the translation block. This is done for internal
emulation state that is rarely accessed directly by the program and/or changes
very often throughout the execution of a translation block---this includes
condition codes on x86, delay slots on SPARC, conditional execution on
ARM, and so on. This state is stored for each target instruction, and
looked up on exceptions.
@item MMU emulation:
For system emulation QEMU uses a software MMU. In that mode, the MMU
virtual to physical address translation is done at every memory
access.
QEMU uses an address translation cache (TLB) to speed up the translation.
In order to avoid flushing the translated code each time the MMU
mappings change, all caches in QEMU are physically indexed. This
means that each basic block is indexed with its physical address.
In order to avoid invalidating the basic block chain when MMU mappings
change, chaining is only performed when the destination of the jump
shares a page with the basic block that is performing the jump.
The MMU can also distinguish RAM and ROM memory areas from MMIO memory
areas. Access is faster for RAM and ROM because the translation cache also
hosts the offset between guest address and host memory. Accessing MMIO
memory areas instead calls out to C code for device emulation.
Finally, the MMU helps tracking dirty pages and pages pointed to by
translation blocks.
@end table
@node QEMU compared to other emulators
@section QEMU compared to other emulators
Like bochs [1], 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 [3] 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 [4]).
TWIN from Willows Software was 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 has 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 [5] 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 Plex86 [6] PC virtualizer is done in the same spirit as the now
obsolete 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 privileged 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 [7], VirtualPC [8]) are faster
than QEMU (without virtualization), but they all need specific, proprietary
and potentially unsafe host drivers. Moreover, they are unable to
provide cycle exact simulation as an emulator can.
VirtualBox [9], Xen [10] and KVM [11] are based on QEMU. QEMU-SystemC
[12] uses QEMU to simulate a system where some hardware devices are
developed in SystemC.
@node Managed start up options
@section Managed start up options
@ -350,59 +196,3 @@ depend on an initialized machine, including but not limited to:
@item query-status
@item x-exit-preconfig
@end table
@node Bibliography
@section Bibliography
@table @asis
@item [1]
@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project,
by Kevin Lawton et al.
@item [2]
@url{http://www.valgrind.org/}, Valgrind, an open-source memory debugger
for GNU/Linux.
@item [3]
@url{http://ftp.dreamtime.org/pub/linux/Linux-Alpha/em86/v0.2/docs/em86.html},
the EM86 x86 emulator on Alpha-Linux.
@item [4]
@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/@/full_papers/chernoff/chernoff.pdf},
DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton
Chernoff and Ray Hookway.
@item [5]
@url{http://user-mode-linux.sourceforge.net/},
The User-mode Linux Kernel.
@item [6]
@url{http://www.plex86.org/},
The new Plex86 project.
@item [7]
@url{http://www.vmware.com/},
The VMWare PC virtualizer.
@item [8]
@url{https://www.microsoft.com/download/details.aspx?id=3702},
The VirtualPC PC virtualizer.
@item [9]
@url{http://virtualbox.org/},
The VirtualBox PC virtualizer.
@item [10]
@url{http://www.xen.org/},
The Xen hypervisor.
@item [11]
@url{http://www.linux-kvm.org/},
Kernel Based Virtual Machine (KVM).
@item [12]
@url{http://www.greensocs.com/projects/QEMUSystemC},
QEMU-SystemC, a hardware co-simulator.
@end table