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On M profile, return from exceptions happen when code in Handler mode executes one of the following function call return instructions: * POP or LDM which loads the PC * LDR to PC * BX register and the new PC value is 0xFFxxxxxx. QEMU tries to implement this by not treating the instruction specially but then catching the attempt to execute from the magic address value. This is not ideal, because: * there are guest visible differences from the architecturally specified behaviour (for instance jumping to 0xFFxxxxxx via a different instruction should not cause an exception return but it will in the QEMU implementation) * we have to account for it in various places (like refusing to take an interrupt if the PC is at a magic value, and making sure that the MPU doesn't deny execution at the magic value addresses) Drop these hacks, and instead implement exception return the way the architecture specifies -- by having the relevant instructions check for the magic value and raise the 'do an exception return' QEMU internal exception immediately. The effect on the generated code is minor: bx lr, old code (and new code for Thread mode): TCG: mov_i32 tmp5,r14 movi_i32 tmp6,$0xfffffffffffffffe and_i32 pc,tmp5,tmp6 movi_i32 tmp6,$0x1 and_i32 tmp5,tmp5,tmp6 st_i32 tmp5,env,$0x218 exit_tb $0x0 set_label $L0 exit_tb $0x7f2aabd61993 x86_64 generated code: 0x7f2aabe87019: mov %ebx,%ebp 0x7f2aabe8701b: and $0xfffffffffffffffe,%ebp 0x7f2aabe8701e: mov %ebp,0x3c(%r14) 0x7f2aabe87022: and $0x1,%ebx 0x7f2aabe87025: mov %ebx,0x218(%r14) 0x7f2aabe8702c: xor %eax,%eax 0x7f2aabe8702e: jmpq 0x7f2aabe7c016 bx lr, new code when in Handler mode: TCG: mov_i32 tmp5,r14 movi_i32 tmp6,$0xfffffffffffffffe and_i32 pc,tmp5,tmp6 movi_i32 tmp6,$0x1 and_i32 tmp5,tmp5,tmp6 st_i32 tmp5,env,$0x218 movi_i32 tmp5,$0xffffffffff000000 brcond_i32 pc,tmp5,geu,$L1 exit_tb $0x0 set_label $L1 movi_i32 tmp5,$0x8 call exception_internal,$0x0,$0,env,tmp5 x86_64 generated code: 0x7fe8fa1264e3: mov %ebp,%ebx 0x7fe8fa1264e5: and $0xfffffffffffffffe,%ebx 0x7fe8fa1264e8: mov %ebx,0x3c(%r14) 0x7fe8fa1264ec: and $0x1,%ebp 0x7fe8fa1264ef: mov %ebp,0x218(%r14) 0x7fe8fa1264f6: cmp $0xff000000,%ebx 0x7fe8fa1264fc: jae 0x7fe8fa126509 0x7fe8fa126502: xor %eax,%eax 0x7fe8fa126504: jmpq 0x7fe8fa122016 0x7fe8fa126509: mov %r14,%rdi 0x7fe8fa12650c: mov $0x8,%esi 0x7fe8fa126511: mov $0x56095dbeccf5,%r10 0x7fe8fa12651b: callq *%r10 which is a difference of one cmp/branch-not-taken. This will be lost in the noise of having to exit generated code and look up the next TB anyway. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <rth@twiddle.net> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org> Message-id: 1491844419-12485-9-git-send-email-peter.maydell@linaro.org
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
Additional information can also be found online via the QEMU website:
http://qemu-project.org/Hosts/Linux
http://qemu-project.org/Hosts/Mac
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