[Development report #7] Improve the kinst DTrace provider
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Date: Mon, 10 Apr 2023 18:42:18 UTC
The past few days I've been working on the forthcoming RISC-V port of kinst. Tracing of individual instructions in kinst so far (i.e in amd64) works by using a trampoline which contains the actual instruction, followed by an unconditional jump back to the next instruction so that we can resume execution. In RISC-V however this turns out to not really be possible, because we cannot encode large displacement values in a single instruction in order to form a far jump [1]. To compensate for this, the RISC-V trampoline contains the instruction, followed by an EBREAK (breakpoint) instruction. kinst_invop() is able to detect if the breakpoint was triggered by the trampoline, where in this case, it manually sets the program counter to the next instruction, instead of using a jump. So far only instructions that do not address memory can be traced without problems. Since the displacement issue I mentioned above re-appears here as well, instructions that need modification [2] will most likely not use the trampoline at all, and be emulated in software instead, like we do in amd64 kinst for call instructions. [1] Trampoline addresses are located above the kernel base address, which can be several MBs away from the instruction we are tracing. [2] Some instructions need modification, that is, for instructions that use relative addressing (e.g PC-relative ones), we have to recalculate the displacement to be relative to the trampoline address.