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rust-embedded / riscv

Low level access to RISC-V processors
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Fix semihosting::exit() on riscv64 QEMU targets. #215

Closed kevin-vigor closed 3 months ago

kevin-vigor commented 3 months ago

QEMU's handler for REPORT_EXCEPTION (a.k.a. TARGET_SYS_EXIT in QEMU sources) expects two arguments for 64-bit systems, including riscv64. In the event that a second argument is not provided, QEMU takes an error path which does not exit the simulation.

The net result is that semihosting::debug::exit() hangs on riscv64 qemu simulation.

Provide the necessry extra paramater to the syscall so that exit now works properly.

Note that the second parameter only affects the simulator exit code if the first parameter is ApplicationExit, and we use ApplicationExit only for successful exit, so we can simply hardcode 0 as second parameter. On the error path we set first parameter to RunTimeErrorUnknown and QEMU properly returns exit code 1 regardless of second parameter.

kevin-vigor commented 3 months ago

Hold off, I just tested this on riscv32 target and return error codes are broken by this change. Will resubmit.

taiki-e commented 3 months ago

SYS_EXIT has the different API on 32-bit and 64-bit:

32-bit

On entry, the PARAMETER register is set to a reason code describing the cause of the trap.

64-bit

On entry, the PARAMETER REGISTER contains a pointer to a two-field argument block:

The master branch is correct for 32-bit and this PR is correct for 64-bit.