Inconsistent `Tn_INT_ROUTE_CAP` and `Tn_INT_ROUTE_CNF` in HPET

Host environment

• Operating system:

  NixOS

• OS/kernel version:

  6.6.71

• Architecture:

  x86_64

• QEMU flavor:

  qemu-system-x86_64

• QEMU version:

  9.20

• QEMU command line:

• OS/kernel version: N/A
• Architecture: x86_64

Description of problem

In the HPET specification it says:

Timer n Interrupt Routing Capability: (where n is the timer number: 00 to 31) This 32-bit read-only field indicates to which interrupts in the I/O (x) APIC this timer’s interrupt can be routed. This is used in conjunction with the Tn_INT_ROUTE_CNF field.

Each bit in this field corresponds to a particular interrupt. For example, if this timer’s interrupt can be mapped to interrupts 16, 18, 20, 22, or 24, then bits 16, 18, 20, 22, and 24 in this field will be set to 1. All other bits will be 0.

Timer n Interrupt Route: (where n is the timer number: 00 to 31). This 5-bit read/write field indicates the routing for the interrupt to the I/O APIC. A maximum value of 32 interrupts are supported. Default is 00h Software writes to this field to select which interrupt in the I/O (x) will be used for this timer’s interrupt. If the value is not supported by this prarticular timer, then the value read back will not match what is written. The software must only write valid values.

In QEMU, the HPET timers indicate that the only I/O APIC IRQ they support is IRQ 2 (based only bit 2 being 1). But actually the HPET interrupt works even if I set it to IRQ 20, which is inconsistent with the Tn_INT_ROUTE_CAP that the timer shows.

The HPET should show that it works with more than just IRQ 2.

Steps to reproduce

1. Git checkout https://github.com/ChocolateLoverRaj/code-runner/tree/fd368f53c1c99885a3b149a59f2959f383f42859
2. nix develop
3. cargo r -- -s -serial mon:stdio --no-reboot -nographic -d int

Additional information