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semihosting: add semihosting section to the docs
The main reason to do this is to document our O_BINARY implementation decision somewhere. However I've also moved some of the implementation details out of qemu-options and added links between the two. As a bonus I've highlighted the scary warnings about host access with the appropriate RST tags. Acked-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Message-Id: <20230124180127.1881110-22-alex.bennee@linaro.org>
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Alex Bennée
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@ -101,3 +101,90 @@ depending on the guest architecture.
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A number of features are are only available when running under
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emulation including :ref:`Record/Replay<replay>` and :ref:`TCG Plugins`.
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.. _Semihosting:
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Semihosting
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-----------
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Semihosting is a feature defined by the owner of the architecture to
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allow programs to interact with a debugging host system. On real
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hardware this is usually provided by an In-circuit emulator (ICE)
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hooked directly to the board. QEMU's implementation allows for
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semihosting calls to be passed to the host system or via the
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``gdbstub``.
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Generally semihosting makes it easier to bring up low level code before a
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more fully functional operating system has been enabled. On QEMU it
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also allows for embedded micro-controller code which typically doesn't
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have a full libc to be run as "bare-metal" code under QEMU's user-mode
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emulation. It is also useful for writing test cases and indeed a
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number of compiler suites as well as QEMU itself use semihosting calls
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to exit test code while reporting the success state.
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Semihosting is only available using TCG emulation. This is because the
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instructions to trigger a semihosting call are typically reserved
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causing most hypervisors to trap and fault on them.
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.. warning::
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Semihosting inherently bypasses any isolation there may be between
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the guest and the host. As a result a program using semihosting can
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happily trash your host system. You should only ever run trusted
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code with semihosting enabled.
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Redirection
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~~~~~~~~~~~
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Semihosting calls can be re-directed to a (potentially remote) gdb
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during debugging via the :ref:`gdbstub<GDB usage>`. Output to the
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semihosting console is configured as a ``chardev`` so can be
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redirected to a file, pipe or socket like any other ``chardev``
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device.
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Supported Targets
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~~~~~~~~~~~~~~~~~
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Most targets offer similar semihosting implementations with some
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minor changes to define the appropriate instruction to encode the
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semihosting call and which registers hold the parameters. They tend to
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presents a simple POSIX-like API which allows your program to read and
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write files, access the console and some other basic interactions.
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For full details of the ABI for a particular target, and the set of
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calls it provides, you should consult the semihosting specification
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for that architecture.
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.. note::
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QEMU makes an implementation decision to implement all file
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access in ``O_BINARY`` mode. The user-visible effect of this is
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regardless of the text/binary mode the program sets QEMU will
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always select a binary mode ensuring no line-terminator conversion
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is performed on input or output. This is because gdb semihosting
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support doesn't make the distinction between the modes and
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magically processing line endings can be confusing.
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.. list-table:: Guest Architectures supporting Semihosting
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:widths: 10 10 80
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:header-rows: 1
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* - Architecture
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- Modes
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- Specification
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* - Arm
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- System and User-mode
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- https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst
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* - m68k
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- System
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- https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=libgloss/m68k/m68k-semi.txt;hb=HEAD
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* - MIPS
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- System
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- Unified Hosting Interface (MD01069)
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* - Nios II
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- System
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- https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=libgloss/nios2/nios2-semi.txt;hb=HEAD
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* - RISC-V
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- System and User-mode
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- https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc
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* - Xtensa
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- System
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- Tensilica ISS SIMCALL
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@ -4633,10 +4633,11 @@ DEF("semihosting", 0, QEMU_OPTION_semihosting,
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QEMU_ARCH_MIPS | QEMU_ARCH_NIOS2 | QEMU_ARCH_RISCV)
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SRST
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``-semihosting``
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Enable semihosting mode (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V only).
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Enable :ref:`Semihosting` mode (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V only).
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Note that this allows guest direct access to the host filesystem, so
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should only be used with a trusted guest OS.
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.. warning::
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Note that this allows guest direct access to the host filesystem, so
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should only be used with a trusted guest OS.
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See the -semihosting-config option documentation for further
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information about the facilities this enables.
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@ -4648,22 +4649,12 @@ QEMU_ARCH_ARM | QEMU_ARCH_M68K | QEMU_ARCH_XTENSA |
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QEMU_ARCH_MIPS | QEMU_ARCH_NIOS2 | QEMU_ARCH_RISCV)
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SRST
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``-semihosting-config [enable=on|off][,target=native|gdb|auto][,chardev=id][,userspace=on|off][,arg=str[,...]]``
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Enable and configure semihosting (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V
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Enable and configure :ref:`Semihosting` (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V
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only).
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Note that this allows guest direct access to the host filesystem, so
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should only be used with a trusted guest OS.
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On Arm this implements the standard semihosting API, version 2.0.
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On M68K this implements the "ColdFire GDB" interface used by
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libgloss.
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Xtensa semihosting provides basic file IO calls, such as
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open/read/write/seek/select. Tensilica baremetal libc for ISS and
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linux platform "sim" use this interface.
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On RISC-V this implements the standard semihosting API, version 0.2.
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.. warning::
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Note that this allows guest direct access to the host filesystem, so
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should only be used with a trusted guest OS.
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``target=native|gdb|auto``
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Defines where the semihosting calls will be addressed, to QEMU
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