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Diffstat (limited to 'docs/user')
| -rw-r--r-- | docs/user/index.rst | 5 | ||||
| -rw-r--r-- | docs/user/main.rst | 60 |
2 files changed, 41 insertions, 24 deletions
diff --git a/docs/user/index.rst b/docs/user/index.rst index 782d27c..2307580 100644 --- a/docs/user/index.rst +++ b/docs/user/index.rst @@ -5,8 +5,9 @@ User Mode Emulation ------------------- This section of the manual is the overall guide for users using QEMU -for user-mode emulation. In this mode, QEMU can launch -processes compiled for one CPU on another CPU. +for user-mode emulation. In this mode, QEMU can launch programs +compiled for one CPU architecture on the same Operating System (OS) +but running on a different CPU architecture. .. toctree:: :maxdepth: 2 diff --git a/docs/user/main.rst b/docs/user/main.rst index 9a1c604..347bdfa 100644 --- a/docs/user/main.rst +++ b/docs/user/main.rst @@ -17,28 +17,44 @@ Features QEMU user space emulation has the following notable features: -**System call translation:** - QEMU includes a generic system call translator. This means that the - parameters of the system calls can be converted to fix endianness and - 32/64-bit mismatches between hosts and targets. IOCTLs can be - converted too. - -**POSIX signal handling:** - QEMU can redirect to the running program all signals coming from the - host (such as ``SIGALRM``), as well as synthesize signals from - virtual CPU exceptions (for example ``SIGFPE`` when the program - executes a division by zero). - - QEMU relies on the host kernel to emulate most signal system calls, - for example to emulate the signal mask. On Linux, QEMU supports both - normal and real-time signals. - -**Threading:** - On Linux, QEMU can emulate the ``clone`` syscall and create a real - host thread (with a separate virtual CPU) for each emulated thread. - Note that not all targets currently emulate atomic operations - correctly. x86 and Arm use a global lock in order to preserve their - semantics. +System call translation +~~~~~~~~~~~~~~~~~~~~~~~ + +System calls are the principle interface between user-space and the +kernel. Generally the same system calls exist on all versions of the +kernel so QEMU includes a generic system call translator. The +translator takes care of adjusting endianess, 32/64 bit parameter size +and then calling the equivalent host system call. + +QEMU can also adjust device specific ``ioctl()`` calls in a similar +fashion. + +POSIX signal handling +~~~~~~~~~~~~~~~~~~~~~ + +QEMU can redirect to the running program all signals coming from the +host (such as ``SIGALRM``), as well as synthesize signals from +virtual CPU exceptions (for example ``SIGFPE`` when the program +executes a division by zero). + +QEMU relies on the host kernel to emulate most signal system calls, +for example to emulate the signal mask. On Linux, QEMU supports both +normal and real-time signals. + +Threading +~~~~~~~~~ + +On Linux, QEMU can emulate the ``clone`` syscall and create a real +host thread (with a separate virtual CPU) for each emulated thread. +However as QEMU relies on the system libc to call ``clone`` on its +behalf we limit the flags accepted to those it uses. Specifically this +means flags affecting namespaces (e.g. container runtimes) are not +supported. QEMU user-mode processes can still be run inside containers +though. + +While QEMU does its best to emulate atomic operations properly +differences between the host and guest memory models can cause issues +for software that makes assumptions about the memory model. QEMU was conceived so that ultimately it can emulate itself. Although it is not very useful, it is an important test to show the power of the |