OSDev.org

Hi,

After a user process calls back kernel (system call using CALL GATE), no IRQs are being fired (particularly TIMER) - while I have configured APIC which works fine before switching to user process

I recently started working on APIC. Though I haven't worked towards supporting multiple processors yet I got APIC working almost
I am able to configure APIC and also local timer to fire TIMER irq (0 -> mapped to 32)
Multiple (kernel) processes which all run in kernel space (ring0) run fine (scheduling works fine switching from one process to another upon timer irq)
However, when I do a task switch to a user process (ring3) and then when it calls back to kernel via a call gate, I notice that timer irq is not firing any more (actually no irqs) - and hence there is no context switch based on irq any more
With the same code base if I just switch back to using PIC then everything works fine

Can anyone please help !

[ This is on QEMU n I'm using hardware task switching ]
Regards,
- Prajwala

As expected, this was some bug in my OS which survived this long but got caught while implementing APIC

Just in case if it helps others

Problem 1) Once I switch to a user process (ring 3, different page table), IRQs stop
Answer: This was a page mapping issue. The first "X" mb of my kernel is mapped 1 to 1 with user process. APIC base (0xFEE00000 and 0xFEC00000) are re-mapped to some area within this kernel space. But while creating page table for user process, I was doing 1 to 1 map without honoring the IO mapped area

Problem 2) On a real PC, there was no keyboard IRQ
Answer: My (now earlier) implementation to consume keyboard characters was to Disable Keyboard IRQ, read them off an internal queue, Enable Keyboard IRQ. This worked fine with PIC probably because by disabling/enabling IRQ, we won't lose the IRQ (?). However, with APIC, when I disabled IRQ, the IRQs are lost (I disable by writing to 0x10 + irqNo * 2 of apic base). On real PC, the foreground process executing this code sequence (i.e. disable KB irq, check/read queue, enable KB irq) was starving APIC and all IRQs were lost (this is my inference). I then changed the approach to use different synchronization approach w/o enable/disable IRQ that worked w/o any issues

Is there a way to disable APIC IRQ without losing them ?