2019-08-15 06:21:51 +00:00
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# `cloud-hypervisor` debug IO port
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`cloud-hypervisor` uses the [`0x80`](https://www.intel.com/content/www/us/en/support/articles/000005500/boards-and-kits.html)
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I/O port to trace user defined guest events.
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Whenever the guest write one byte between `0x0` and `0xF` on this particular
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I/O port, `cloud-hypervisor` will log and timestamp that event at the `debug`
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log level.
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It is up to the guest stack to decide when and what to write to the 0x80 port
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in order to signal the host about specific events and have `cloud-hypervisor`
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log it.
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`cloud-hypervisor` defines several debug port code ranges that should be used
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for debugging specific components of the guest software stack. When logging a
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write of one of those codes to the debug port, `cloud-hypervisor` adds a
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pre-defined string to the logs.
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| Code Range | Component | Log string |
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| ---------------- | ----------- | ------------ |
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| `0x00` to `0x1f` | Firmware | `Firmware` |
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| `0x20` to `0x3f` | Bootloader | `Bootloader` |
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| `0x40` to `0x5f` | Kernel | `Kernel` |
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| `0x60` to `0x7f` | Userspace | `Userspace` |
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| `0x80` to `0xff` | Custom | `Custom` |
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One typical use case is guest boot time measurement and tracing. By writing
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different values to the debug I/O port at different boot process steps, the
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guest will have `cloud-hypervisor` generate timestamped logs of all those steps.
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That provides a basic but convenient way of measuring not only the overall guest
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boot time but all intermediate steps as well.
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## Logging
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Assuming parts of the guest software stack have been instrumented to use the
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`cloud-hypervisor` debug I/O port, we may want to gather the related logs.
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To do so we need to start `cloud-hypervisor` with the right debug level
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(`-vvv`). It is also recommended to have it log into a dedicated file in order
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to easily grep for the tracing logs (e.g.
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`--log-file /tmp/cloud-hypervisor.log`):
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```
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./target/debug/cloud-hypervisor \
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--kernel ~/rust-hypervisor-firmware/target/target/release/hypervisor-fw \
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2020-07-02 16:30:16 +00:00
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--disk path=~/hypervisor/images/focal-server-cloudimg-amd64.raw \
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2019-08-15 06:21:51 +00:00
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--cpus 4 \
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--memory size=1024M \
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--rng \
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--log-file /tmp/ch-fw.log \
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-vvv
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```
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After booting the guest, we then have to grep for the debug I/O port traces in
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the log file:
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```Shell
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$ grep "Debug I/O port" /tmp/ch-fw.log
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cloud-hypervisor: 19.762449ms: DEBUG:vmm/src/vm.rs:510 -- [Debug I/O port: Firmware code 0x0] 0.019004 seconds
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cloud-hypervisor: 403.499628ms: DEBUG:vmm/src/vm.rs:510 -- [Debug I/O port: Firmware code 0x1] 0.402744 seconds
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```
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