68 lines
3.5 KiB
Markdown
68 lines
3.5 KiB
Markdown
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## Why is my execution so slow?
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Typically, it’s due to
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- Instrumenting every instruction executed.
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- Instrumenting every memory access.
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Optimize your program with less instrumentation.
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## Why do I get a wrong PC after emulation stops?
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PC is only guaranteed to be correct if you install `UC_HOOK_CODE`. This is due to the fact that updating PC is a big performance overhead during emulation.
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## I get an “Unhandled CPU Exception”, why?
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Unicorn is a pure CPU emulator and usually it’s due to no handler registered for instructions like `syscall` and `SVC`. If you expect system emulation, you probably would like [qiling framework](https://github.com/qilingframework/qiling).
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## I would like to instrument a specific instruction but get a `UC_ERR_HOOK`, why?
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Currently, only a small subset of the instructions can be instrumented.
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On x86, all available instructions are: `in` `out` `syscall` `sysenter` `cpuid`.
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## Emulating some instructions gives an error, what should I do?
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1. Some instructions are not enabled by default on some architectures. For example, you have to setup CSR on RISC-V or VFP on ARM before emulating floating-point instructions. Refer to the corresponding manual to check if you leave out possible switches in special registers.
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2. If you are on ARM, please check whether you are emulating a THUMB instruction. If so, please use `UC_MODE_THUMB` and make sure the starting address is odd.
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3. If either is not the case, it might be some newer instruction sets that qemu5 doesn’t support.
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If you are still using Unicorn1, please upgrade to Unicorn2 for better support.
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## I can't recover from unmapped read/write even I return `true` in the hook, why?
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This is a minor change in memory hooks behavior between Unicorn1 and Unicorn2. To gracefully recover from memory read/write error, you have to map the invalid memory before you return true.
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It is due to the fact that, if users return `true` without memory mapping set up correctly, we don't know what to do next. In Unicorn1, the behavior is __undefined__ in this case but in Unicorn2 we would like to force users to set up memory mapping in the hook to continue execution.
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See the [sample](https://github.com/unicorn-engine/unicorn/blob/c05fbb7e63aed0b60fc2888e08beceb17bce8ac4/samples/sample_x86.c#L1379-L1393) for details.
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## How to emulate interrupts (or ticks) with Unicorn?
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As stated, Unicorn is a pure CPU emulator. For such emulation, you have two choices:
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- Use the `timeout` parameter of `uc_emu_start`
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- Use the `count` parameter of `uc_emu_start`
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After emulation stops, you may check anything you feel interested and resume emulation accordingly.
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Note that for cortex-m `exec_return`, Unicorn has a magic software exception with interrupt number 8. You may register a hook to handle that.
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## Why not keep up the upstream qemu?
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To provide end users with simple API, Unicorn does lots of dirty hacks within qemu code which prevents it from sync painlessly.
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## Is there anyway to disable softmmu to speed up execution?
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Yes, it’s possible but that is not Unicorn’s goal and there is no simple switch in qemu to disable softmmu.
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## I'd like to make contributions, where do I start?
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See [milestones](https://github.com/unicorn-engine/unicorn/milestones) and [coding convention](https://github.com/unicorn-engine/unicorn/wiki/Coding-Convention
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).
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Be sure to send pull requests for our **dev** branch only.
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## Which qemu version is Unicorn based on?
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Prior to 2.0.0, Unicorn is based on qemu 2.2.1. After that, Unicorn is based on qemu 5.0.1.
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