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Here is the description of the mcrfs instruction from the PowerPC Architecture Book, Version 2.02, Book I: PowerPC User Instruction Set Architecture (http://www.ibm.com/developerworks/systems/library/es-archguide-v2.html), found on page 120: The contents of FPSCR field BFA are copied to Condition Register field BF. All exception bits copied are set to 0 in the FPSCR. If the FX bit is copied, it is set to 0 in the FPSCR. Special Registers Altered: CR field BF FX OX (if BFA=0) UX ZX XX VXSNAN (if BFA=1) VXISI VXIDI VXZDZ VXIMZ (if BFA=2) VXVC (if BFA=3) VXSOFT VXSQRT VXCVI (if BFA=5) However, currently every bit in FPSCR field BFA is set to 0, including ones not on that list. This can be seen in the following simple C program: #include <fenv.h> #include <stdio.h> int main(int argc, char **argv) { int ret; ret = fegetround(); printf("Current rounding: %d\n", ret); ret = fesetround(FE_UPWARD); printf("Setting to FE_UPWARD (%d): %d\n", FE_UPWARD, ret); ret = fegetround(); printf("Current rounding: %d\n", ret); ret = fegetround(); printf("Current rounding: %d\n", ret); return 0; } which gave the output (before this commit): Current rounding: 0 Setting to FE_UPWARD (2): 0 Current rounding: 2 Current rounding: 0 instead of (after this commit): Current rounding: 0 Setting to FE_UPWARD (2): 0 Current rounding: 2 Current rounding: 2 The relevant disassembly is in fegetround(), which, on my system, is: __GI___fegetround: <+0>: mcrfs cr7, cr7 <+4>: mfcr r3 <+8>: clrldi r3, r3, 62 <+12>: blr What happens is that, the first time fegetround() is called, FPSCR field 7 is retrieved. However, because of the bug in mcrfs, the entirety of field 7 is set to 0, which includes the rounding mode. There are other issues this will fix, such as condition flags not persisting when they should if read, and if you were to read a specific field with some exception bits set, but no others were set in the entire register, then the bits would be cleared correctly, but FEX/VX would not be updated to 0 as they should be. Signed-off-by: James Clarke <jrtc27@jrtc27.com> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Complete details of the process for building and configuring QEMU for
all supported host platforms can be found in the qemu-tech.html file.
Additional information can also be found online via the QEMU website:
http://qemu-project.org/Hosts/Linux
http://qemu-project.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu-project.org/qemu.git
When submitting patches, the preferred approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via
the QEMU website
http://qemu-project.org/Contribute/SubmitAPatch
http://qemu-project.org/Contribute/TrivialPatches
Bug reporting
=============
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.
For additional information on bug reporting consult:
http://qemu-project.org/Contribute/ReportABug
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
- qemu-devel@nongnu.org
http://lists.nongnu.org/mailman/listinfo/qemu-devel
- #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
http://qemu-project.org/Contribute/StartHere
-- End