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cloud-hypervisor debug IO port

cloud-hypervisor uses the 0x80 I/O port to trace user defined guest events.

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 log level.

It is up to the guest stack to decide when and what to write to the 0x80 port in order to signal the host about specific events and have cloud-hypervisor log it.

cloud-hypervisor defines several debug port code ranges that should be used for debugging specific components of the guest software stack. When logging a write of one of those codes to the debug port, cloud-hypervisor adds a pre-defined string to the logs.

Code Range Component Log string
0x00 to 0x1f Firmware Firmware
0x20 to 0x3f Bootloader Bootloader
0x40 to 0x5f Kernel Kernel
0x60 to 0x7f Userspace Userspace
0x80 to 0xff Custom Custom

One typical use case is guest boot time measurement and tracing. By writing different values to the debug I/O port at different boot process steps, the 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

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.

To do so we need to start cloud-hypervisor with the right debug level (-v -v -v). It is also recommended to have it log into a dedicated file in order to easily grep for the tracing logs (e.g. --log-file /tmp/cloud-hypervisor.log):

./target/debug/cloud-hypervisor \
    --kernel ~/rust-hypervisor-firmware/target/target/release/hypervisor-fw \
    --disk path=~/hypervisor/images/focal-server-cloudimg-amd64.raw \
    --cpus 4 \
    --memory size=1024M \
    --rng \
    --log-file /tmp/ch-fw.log \
    -v -v -v

After booting the guest, we then have to grep for the debug I/O port traces in the log file:

$ 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