On Wed, 27 Apr 2022 at 23:27, Stafford Horne <sho...@gmail.com> wrote: > Yes, this is what was brought up before. At that time semihosting was > mentioned > and I tried to understand what it was but didn't really understand it as a > general > concept. Is this something arm specific?
QEMU uses "semihosting" for the general concept of a syscall-like ABI provided by the model that allows guest code written to use it to access some facilities as if it were a program running on the host rather than running on bare metal. (I think the term derives originally from the Arm term for this kind of functionality, but the concept is not Arm-specific.) Arm defines an ABI which looks basically like a set of syscalls: code sets up some registers and executes a specific SVC or HLT or other magic instruction, and the implementation is supposed to then act on that. You can do things like "open file", "read file", "exit", etc. https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst The idea is that simulators and also debug stubs or debuggers can implement this, and then bare-metal code can be written to use it, mainly for debugging and test case purposes. The risc-v folks decided they needed similar functionality, and that the easiest way to do this was to align with the Arm specification and document the risc-v specific bits: https://github.com/riscv/riscv-semihosting-spec Some other architectures have an equivalent thing but which isn't the same set of functions as the Arm version; eg the nios2 version is documented here: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=libgloss/nios2/nios2-semi.txt;hb=HEAD > Since the qemu or1k-sim defines our "simulator", I suspect I could add a > definition of our simulator ABI to the OpenRISC architecture specification. > The > simulation uses of l.nop N as ABI hooks is a de-facto standard for OpenRISC. > From the way you describe this now I take it if we document this as a > architecture simulation ABI the patch would be accepted. If it's something that (a) is documented officially somewhere and (b) everybody is using consistently (ie guest code such as GNU newlib, QEMU, other simulators, etc), then yes, that's OK. It sounds like you just need to write down the details of your de-facto standard to turn it into a de-jure one :-) We would want to guard it behind the existing semihosting command line option, rather than having it enabled all the time, but that part should be straightforward. -- PMM
