Re: [Qemu-devel] [PATCH] Fix timer interrupt routing for non-ACPI guest

[Beth Kon <eak@us.ibm.com>]

Ed Swierk wrote:
> Qemu 0.10.2 is unable to boot a non-ACPI kernel due to a BIOS bug:
>
> ENABLING IO-APIC IRQs 
> ..TIMER: vector=0x31 apic1=0 pin1=0 apic2=-1 pin2=-1 
> ..MP-BIOS bug: 8254 timer not connected to IO-APIC 
> ...trying to set up timer (IRQ0) through the 8259A ... 
> ..... (found apic 0 pin 0) ... 
> ....... failed. 
> ...trying to set up timer as Virtual Wire IRQ... 
>
> The interrupt routing table in the MPTABLE needs to route the timer
> interrupt (IRQ 0) to IOAPIC pin 2.
>
> A similar bug was recently fixed in the ACPI table code.  This patch
> fixes the problem for non-ACPI guests.
>
> Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>
>
> ---
> diff --git a/bios/rombios32.c b/bios/rombios32.c
> index 7be4216..13d5435 100644
> --- a/bios/rombios32.c
> +++ b/bios/rombios32.c
> @@ -1173,7 +1173,7 @@ static void mptable_init(void)
>          putb(&q, 0); /* flags: po=0, el=0 */
>          putb(&q, 0);
>          putb(&q, 0); /* source bus ID = ISA */
> -        putb(&q, i); /* source bus IRQ */
> +        putb(&q, i == 2 ? 0 : i); /* source bus IRQ */
>          putb(&q, ioapic_id); /* dest I/O APIC ID */
>          putb(&q, i); /* dest I/O APIC interrupt in */
>      }
>
>
>
>   
This was an oversight on my part when I submitted the HPET patches for 
qemu. I properly modified the ACPI configuration, but missed the 
non-ACPI case.

This patch is not quite correct and I will submit the necessary patch 
shortly.

A bit of the history...

This issue surfaced when I implemented the HPET and discovered that 
Win2k8 would only work if the HPET interrupt was routed to IOAPIC inti2. 
This is a common configuration (overriding irq0->inti2). The HPET spec 
states that "Timer 0 will be routed to IRQ0 in the Non-APIC or IRQ2 in 
the I/O APIC", and Windows appears to expect it regardless of how 
overrides are reported by the BIOS. So the decision was made to always 
have the override.