[Qemu-devel] [PATCH 0/2] qemu-tech: move part to docs/devel, delete part

[Qemu-devel] [PATCH 0/2] qemu-tech: move part to docs/devel, delete part

[Peter Maydell]

This patchset makes some of the cleanups to qemu-tech.texi which
I suggested in my Sphinx transition plan:
https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg04932.html

(1) the "translator internals" docs move to the devel/ manual
(and are given a simple by-hand texi-to-rst conversion)
(2) the "compared to other emulators" section is simply deleted:
since we haven't updated it since 2015 we obviously don't
care enough about it to keep it useful for users, and the wiki
or website would be a better place for that kind of "you might
want to know this before you install" information if we did
have anybody interested in keeping it current.

I suspect the TCG docs may have some stale info in them too,
but the devel/ manual isn't user-facing so not a very big deal.
Followup patches correcting any inaccuracies welcome :-)

thanks
-- PMM

Peter Maydell (2):
  Convert "translator internals" docs to RST, move to devel manual
  qemu-tech.texi: Remove "QEMU compared to other emulators" section

 docs/devel/index.rst |   1 +
 docs/devel/tcg.rst   | 111 +++++++++++++++++++++++
 qemu-tech.texi       | 210 -------------------------------------------
 3 files changed, 112 insertions(+), 210 deletions(-)
 create mode 100644 docs/devel/tcg.rst

-- 
2.20.1

[Qemu-devel] [PATCH 1/2] Convert "translator internals" docs to RST, move to devel manual

[Peter Maydell]

Our user-facing manual currently has a section "translator internals"
which has some high-level information about the design of the
TCG translator. This should really be in our new devel/ manual.
Convert it to RST format and move it there.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 docs/devel/index.rst |   1 +
 docs/devel/tcg.rst   | 111 +++++++++++++++++++++++++++++++++++++++++++
 qemu-tech.texi       | 103 ---------------------------------------
 3 files changed, 112 insertions(+), 103 deletions(-)
 create mode 100644 docs/devel/tcg.rst

diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index 2a4ddf40ad3..1ec61fcfed9 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -21,3 +21,4 @@ Contents:
    testing
    decodetree
    secure-coding-practices
+   tcg
diff --git a/docs/devel/tcg.rst b/docs/devel/tcg.rst
new file mode 100644
index 00000000000..4956a30a4e6
--- /dev/null
+++ b/docs/devel/tcg.rst
@@ -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.
+
diff --git a/qemu-tech.texi b/qemu-tech.texi
index 7c3d1f05e13..eb430daacf2 100644
--- a/qemu-tech.texi
+++ b/qemu-tech.texi
@@ -161,109 +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
 
-- 
2.20.1

[Qemu-devel] [PATCH 2/2] qemu-tech.texi: Remove "QEMU compared to other emulators" section

[Peter Maydell]

The "QEMU compared to other emulators" section of our documentation
hasn't been updated since 2015 (and parts of the text are even older).
We're clearly not very well placed to track the evolution of a
dozen other emulation projects, and an inaccurate or out of date
comparison doesn't serve anybody, so we're best off just removing
the whole documentation section.

If anybody cares strongly about maintaining a comparison page,
it's probably better to do that on the project's wiki where
we can update it more dynamically.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 qemu-tech.texi | 107 -------------------------------------------------
 1 file changed, 107 deletions(-)

diff --git a/qemu-tech.texi b/qemu-tech.texi
index eb430daacf2..3451cfaa5be 100644
--- a/qemu-tech.texi
+++ b/qemu-tech.texi
@@ -161,57 +161,6 @@ may be created from overlay with minimal amount of hand-written code.
 
 @end itemize
 
-@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
 
@@ -247,59 +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
-- 
2.20.1

Re: [Qemu-devel] [PATCH 2/2] qemu-tech.texi: Remove "QEMU compared to other emulators" section

[Daniel P. Berrangé]

On Fri, Jun 07, 2019 at 04:28:27PM +0100, Peter Maydell wrote:
> The "QEMU compared to other emulators" section of our documentation
> hasn't been updated since 2015 (and parts of the text are even older).
> We're clearly not very well placed to track the evolution of a
> dozen other emulation projects, and an inaccurate or out of date
> comparison doesn't serve anybody, so we're best off just removing
> the whole documentation section.
> 
> If anybody cares strongly about maintaining a comparison page,
> it's probably better to do that on the project's wiki where
> we can update it more dynamically.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  qemu-tech.texi | 107 -------------------------------------------------
>  1 file changed, 107 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>



Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

Re: [Qemu-devel] [PATCH 0/2] qemu-tech: move part to docs/devel, delete part

[Richard Henderson]

On 6/7/19 10:28 AM, Peter Maydell wrote:
> Peter Maydell (2):
>   Convert "translator internals" docs to RST, move to devel manual
>   qemu-tech.texi: Remove "QEMU compared to other emulators" section

Acked-by: Richard Henderson <richard.henderson@linaro.org>


r~

Re: [Qemu-devel] [PATCH 0/2] qemu-tech: move part to docs/devel, delete part

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1485 bytes --]

On Fri, Jun 07, 2019 at 04:28:25PM +0100, Peter Maydell wrote:
> This patchset makes some of the cleanups to qemu-tech.texi which
> I suggested in my Sphinx transition plan:
> https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg04932.html
> 
> (1) the "translator internals" docs move to the devel/ manual
> (and are given a simple by-hand texi-to-rst conversion)
> (2) the "compared to other emulators" section is simply deleted:
> since we haven't updated it since 2015 we obviously don't
> care enough about it to keep it useful for users, and the wiki
> or website would be a better place for that kind of "you might
> want to know this before you install" information if we did
> have anybody interested in keeping it current.
> 
> I suspect the TCG docs may have some stale info in them too,
> but the devel/ manual isn't user-facing so not a very big deal.
> Followup patches correcting any inaccuracies welcome :-)
> 
> thanks
> -- PMM
> 
> Peter Maydell (2):
>   Convert "translator internals" docs to RST, move to devel manual
>   qemu-tech.texi: Remove "QEMU compared to other emulators" section
> 
>  docs/devel/index.rst |   1 +
>  docs/devel/tcg.rst   | 111 +++++++++++++++++++++++
>  qemu-tech.texi       | 210 -------------------------------------------
>  3 files changed, 112 insertions(+), 210 deletions(-)
>  create mode 100644 docs/devel/tcg.rst
> 
> -- 
> 2.20.1
> 
> 

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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