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| -rw-r--r-- | docs/about/emulation.rst | 87 | ||||
| -rw-r--r-- | qemu-options.hx | 25 |
2 files changed, 95 insertions, 17 deletions
diff --git a/docs/about/emulation.rst b/docs/about/emulation.rst index 7ec7e09..b510a54 100644 --- a/docs/about/emulation.rst +++ b/docs/about/emulation.rst @@ -101,3 +101,90 @@ depending on the guest architecture. A number of features are are only available when running under emulation including :ref:`Record/Replay<replay>` and :ref:`TCG Plugins`. + +.. _Semihosting: + +Semihosting +----------- + +Semihosting is a feature defined by the owner of the architecture to +allow programs to interact with a debugging host system. On real +hardware this is usually provided by an In-circuit emulator (ICE) +hooked directly to the board. QEMU's implementation allows for +semihosting calls to be passed to the host system or via the +``gdbstub``. + +Generally semihosting makes it easier to bring up low level code before a +more fully functional operating system has been enabled. On QEMU it +also allows for embedded micro-controller code which typically doesn't +have a full libc to be run as "bare-metal" code under QEMU's user-mode +emulation. It is also useful for writing test cases and indeed a +number of compiler suites as well as QEMU itself use semihosting calls +to exit test code while reporting the success state. + +Semihosting is only available using TCG emulation. This is because the +instructions to trigger a semihosting call are typically reserved +causing most hypervisors to trap and fault on them. + +.. warning:: + Semihosting inherently bypasses any isolation there may be between + the guest and the host. As a result a program using semihosting can + happily trash your host system. You should only ever run trusted + code with semihosting enabled. + +Redirection +~~~~~~~~~~~ + +Semihosting calls can be re-directed to a (potentially remote) gdb +during debugging via the :ref:`gdbstub<GDB usage>`. Output to the +semihosting console is configured as a ``chardev`` so can be +redirected to a file, pipe or socket like any other ``chardev`` +device. + +Supported Targets +~~~~~~~~~~~~~~~~~ + +Most targets offer similar semihosting implementations with some +minor changes to define the appropriate instruction to encode the +semihosting call and which registers hold the parameters. They tend to +presents a simple POSIX-like API which allows your program to read and +write files, access the console and some other basic interactions. + +For full details of the ABI for a particular target, and the set of +calls it provides, you should consult the semihosting specification +for that architecture. + +.. note:: + QEMU makes an implementation decision to implement all file + access in ``O_BINARY`` mode. The user-visible effect of this is + regardless of the text/binary mode the program sets QEMU will + always select a binary mode ensuring no line-terminator conversion + is performed on input or output. This is because gdb semihosting + support doesn't make the distinction between the modes and + magically processing line endings can be confusing. + +.. list-table:: Guest Architectures supporting Semihosting + :widths: 10 10 80 + :header-rows: 1 + + * - Architecture + - Modes + - Specification + * - Arm + - System and User-mode + - https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst + * - m68k + - System + - https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=libgloss/m68k/m68k-semi.txt;hb=HEAD + * - MIPS + - System + - Unified Hosting Interface (MD01069) + * - Nios II + - System + - https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=libgloss/nios2/nios2-semi.txt;hb=HEAD + * - RISC-V + - System and User-mode + - https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc + * - Xtensa + - System + - Tensilica ISS SIMCALL diff --git a/qemu-options.hx b/qemu-options.hx index d59d197..88e93c6 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4633,10 +4633,11 @@ DEF("semihosting", 0, QEMU_OPTION_semihosting, QEMU_ARCH_MIPS | QEMU_ARCH_NIOS2 | QEMU_ARCH_RISCV) SRST ``-semihosting`` - Enable semihosting mode (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V only). + Enable :ref:`Semihosting` mode (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V only). - Note that this allows guest direct access to the host filesystem, so - should only be used with a trusted guest OS. + .. warning:: + Note that this allows guest direct access to the host filesystem, so + should only be used with a trusted guest OS. See the -semihosting-config option documentation for further information about the facilities this enables. @@ -4648,22 +4649,12 @@ QEMU_ARCH_ARM | QEMU_ARCH_M68K | QEMU_ARCH_XTENSA | QEMU_ARCH_MIPS | QEMU_ARCH_NIOS2 | QEMU_ARCH_RISCV) SRST ``-semihosting-config [enable=on|off][,target=native|gdb|auto][,chardev=id][,userspace=on|off][,arg=str[,...]]`` - Enable and configure semihosting (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V + Enable and configure :ref:`Semihosting` (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V only). - Note that this allows guest direct access to the host filesystem, so - should only be used with a trusted guest OS. - - On Arm this implements the standard semihosting API, version 2.0. - - On M68K this implements the "ColdFire GDB" interface used by - libgloss. - - Xtensa semihosting provides basic file IO calls, such as - open/read/write/seek/select. Tensilica baremetal libc for ISS and - linux platform "sim" use this interface. - - On RISC-V this implements the standard semihosting API, version 0.2. + .. warning:: + Note that this allows guest direct access to the host filesystem, so + should only be used with a trusted guest OS. ``target=native|gdb|auto`` Defines where the semihosting calls will be addressed, to QEMU |