Fixed the slavio timer bug properly

Fixed the timer bug found last week more correctly. Will submit the patch later.

Gosh, it is hot over here. Today is something like 38C (~100F) . The cpu in my head is refusing to send any patches and fix any further bugs till the temperature drops.

Fixed another bug in the slavio timer emulation

Trying to get NeXTStep/sparc to boot without any success, I got back to the old bug which seemed to be related: some versions of OBP hang at boot waiting for the timer interrupts.

Somehow I got poisoned by the motto of the qemu developers: OBP doesn't work because the initial set-up is wrong. On the real hardware no one would expect BIOS to work if the machine doesn't pass the power-on self test (POST). But then I it came to me that exactly this motto prevented other people to get OBP working under qemu-system-sparc.

So, I went on and asked Mitch, if he thought whether his creation - OBP - was buggy and relied on the [probably missing] POST initialization. Mitch said that he's pretty much sure that the OBP would do the right thing in this case, so I took another look at the qemu timer code, and fixed the bug.

The bug turned out to be unrelated to the NeXTStep boot problem. On the other hand the fix provides the alternatives to SPARCStation-5 emulation. Now it's possible to get SPARCStation-10 firmware to work, which gives 512m to the guest.

Here come some boot logs with the OBP from SPARCStation-10 and LX.

Yet another bug in IRQ emulation

Trying to find out why NetBSD versions 1.6-3.1 do not boot, I found a bug in IRQ processing. After fixing it these versions still don't boot, :) but fail more gracefully.
Looks like there is another couple of bugs to go...

Spurious interrupts

Previously qemu dropped interrupts on disabling them. The real hardware doesn't do it. Which means, that lots of interrupts were dropped, including the spurious ones. But the real ones were dropped too, that's why the system timer was ticking so slow.

The question is where the spurious  IRQs are coming from: it's not only the ESP which produces them, under Solaris 8 & 9 there are lots of complains about spurious timer interrupts, and they both seem to crash due to a buffer overflow during processing of a (possibly spurious) LE interrupt.

NetBSD 1.3.3 boot doesn't crash with my wrong irq patch. The patch makes qemu drop interrupts, including the spurious ones.

All in all it looked like there is one global problem with interrupts processing. Until now. But now it looks like there is not one source of spurious interrupts, but many.

• esp doesn't seem to produce spurious interrupts under Solaris while reading.
• I've found a bug in the slavio timer which produces spurious interrupts.
• NetBSD may be crashing due to another issue: I don't have a disk which I could boot under OBP. It is possible that OpenBIOS is not compatible with the older NetBSD versions. Update: This is the reason for NetBSD crashing, Michael Kostylev confirmed it.