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devices: add debug-console device

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This commit adds the debug-console (or debugcon) device to CHV. It is a
very simple device on I/O port 0xe9 supported by QEMU and BOCHS. It is
meant for printing information as easy as possible, without any
necessary configuration from the guest at all.

It is primarily interesting to OS/kernel and firmware developers as they
can produce output as soon as the guest starts without any configuration
of a serial device or similar. Furthermore, a kernel hacker might use
this device for information of type B whereas information of type A are
printed to the serial device.

This device is not used by default by Linux, Windows, or any other
"real" OS, but only by toy kernels and during firmware development.

In the CLI, it can be configured similar to --console or --serial with
the --debug-console parameter.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
This commit is contained in:
Philipp Schuster
2024-01-17 22:12:43 +01:00
committed by Bo Chen
parent 8f90fba250
commit e50a641126
11 changed files with 429 additions and 14 deletions
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43
docs/debug-port.md
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@@ -1,7 +1,16 @@
# `cloud-hypervisor` debug IO port
# `cloud-hypervisor` debug IO ports
`cloud-hypervisor` uses the [`0x80`](https://www.intel.com/content/www/us/en/support/articles/000005500/boards-and-kits.html)
I/O port to trace user defined guest events.
When running x86 guests, `cloud-hypervisor` provides different kinds of debug ports:
- [`0x80` debug port](https://www.intel.com/content/www/us/en/support/articles/000005500/boards-and-kits.html)
- Debug console (by default at `0xe9`).
- Firmware debug port at `0x402`.
All of them can be used to trace user-defined guest events and all of them can
be used simultaneously.
## Debug Ports Overview
### `0x80` I/O port
Whenever the guest write one byte between `0x0` and `0xF` on this particular
I/O port, `cloud-hypervisor` will log and timestamp that event at the `debug`
@@ -30,7 +39,7 @@ guest will have `cloud-hypervisor` generate timestamped logs of all those steps.
That provides a basic but convenient way of measuring not only the overall guest
boot time but all intermediate steps as well.
## Logging
#### Logging
Assuming parts of the guest software stack have been instrumented to use the
`cloud-hypervisor` debug I/O port, we may want to gather the related logs.
@@ -59,3 +68,29 @@ $ grep "Debug I/O port" /tmp/ch-fw.log
cloud-hypervisor: 19.762449ms: DEBUG:vmm/src/vm.rs:510 -- [Debug I/O port: Firmware code 0x0] 0.019004 seconds
cloud-hypervisor: 403.499628ms: DEBUG:vmm/src/vm.rs:510 -- [Debug I/O port: Firmware code 0x1] 0.402744 seconds
```
### Debug console port
The debug console is inspired by QEMU and Bochs, which have a similar feature.
By default, the I/O port `0xe9` is used. This port can be configured like a
console. Thus, it can print to a tty, a file, or a pty, for example.
### Firmware debug port
The firmware debug port is also a simple port that prints all bytes written to
it. The firmware debug port only prints to stdout.
## When do I need these ports?
The ports are on the one hand interesting for firmware or kernel developers, as
they provide an easy way to print debug information from within a guest.
Furthermore, you can patch "normal" software to measure certain events, such as
the boot time of a guest.
## Which port should I choose?
The `0x80` debug port and the port of the firmware debug device are always
available. The debug console must be activated via the command line, but
provides more configuration options.
You can use different ports for different aspect of your logging messages.