[Qemu-devel] debugging apic

[Sam King <kingst@uiuc.edu>]

[-- Attachment #1: Type: text/plain, Size: 2103 bytes --]

  Hello,

I am seeing a weird crash in my system and I am trying to figure out if 
it is a software bug or a qemu emulation bug.  From the software 
perspective I am getting a GP fault at a time where it looks like 
everything should be running normally.  After digging into the Qemu 
source code I found out where the GPF was coming from.  It looks like 
intno = -1 when it was being passed into do_interrupt64, which was 
triggering one of the GPF checks.  From what I can tell, intno was being 
set to -1 by an interrupt_request in cpu-exec.c, which was going down 
the following if statement around line 409 of that file:

else if ((interrupt_request & CPU_INTERRUPT_HARD) &&
                                    (((env->hflags2 & HF2_VINTR_MASK) &&
                                      (env->hflags2 & HF2_HIF_MASK)) ||
                                     (!(env->hflags2 & HF2_VINTR_MASK) &&
                                      (env->eflags & IF_MASK &&
                                       !(env->hflags & 
HF_INHIBIT_IRQ_MASK)))))

and from within that else if statement, env has the following state:

hflags2 = 0x00000001
eflags = 0x00003202
hflags = 0x0040c0b7
interrupt request = 0x00000002

But intno is being set equal to -1 by the call to cpu_get_pic_interrupt, 
from the call to apic_accept_pic_intr returning 0.  If I change the 
cpu_get_pic_interrupt code to this:

int cpu_get_pic_interrupt(CPUState *env)
{
     int intno;

     intno = apic_get_interrupt(env);
     if (intno >= 0) {
         /* set irq request if a PIC irq is still pending */
         /* XXX: improve that */
         pic_update_irq(isa_pic);
         return intno;
     }
     /* read the irq from the PIC */
     if (!apic_accept_pic_intr(env)) {
         //return -1;
     }

     intno = pic_read_irq(isa_pic);

     return intno;
}

Then the issue manifests as a spurious interrupt and the software 
ignores it, avoiding the GPF.  Does anyone have any ideas as to what is 
going wrong here?  Should I look more closely at the Qemu emulation code 
or my software? Any help is appreciated.

Thanks!

--Sam

[-- Attachment #2: Type: text/html, Size: 4237 bytes --]