id = 2007
title = "Unable to update APIC_TPR when x2APIC is enabled and -global kvm-pit.lost_tick_policy=discard parameter provided"
state = "opened"
created_at = "2023-11-27T13:58:54.734Z"
closed_at = "n/a"
labels = ["accel: KVM", "target: i386"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2007"
host-os = "Gentoo Linux"
host-arch = "x86"
qemu-version = "QEMU emulator version 8.1.2"
guest-os = "OSDEV (custom OS)"
guest-arch = "x86 (both i686 and amd64)"
description = """I am developing a custom OS and I wanted to implement x2APIC support. I was able to enable x2APIC, read and write some registers, like APIC_VER and APIC_SIVR. Everything looks good, except that I cannot update APIC_TPR register. Reading it always returns 0. The code I wrote works properly on bare metal. Below some observations:

Scenario 1:
1. Enable x2APIC
2. Write to CR8 - success
3. Read from CR8 - gives correct value
4. Read from APIC_TPR - gives correct value

Scenario 2:
1. Enable x2APIC
2. Read from APIC_TPR - gives 0
3. Write to APIC_TPR
4. Read from APIC_TPR - gives 0 again

Scenario 3:
1. Initialize APIC (LAPIC or xAPIC)
2. Write to APIC_TPR
3. Read from APIC_TPR - gives correct value
4. Switch to x2APIC
5. Read from APIC_TPR - gives correct value stored in pt. 2
6. Write to APIC_TPR
7. Read from APIC_TPR - gives values stored in pt.2, not in point 6!

Looks like APIC_TPR is stuck at value stored there before switching to x2APIC and it cannot be updated with MSR. Only update CR8 works.
I have checked parameters I passed to qemu. After removing `-global kvm-pit.lost_tick_policy=discard` problem is gone and APIC_TPR is updated correctly."""
reproduce = "n/a"
additional = """Please let me know if you need additional information."""