Re: [PATCH] hw/intc/ioapic: Delete a wrong IRQ redirection on I/O APIC

[Paolo Bonzini]

On Tue, Jun 25, 2024 at 2:03 PM TaiseiIto <taisei1212@outlook.jp> wrote:
> Before this commit, interruptions from i8254 which should be sent to IRQ0
> were sent to IRQ2. After this commit, these are correctly sent to IRQ0. When
> I had an HPET timer generate interruptions once per second to test an HPET
> driver in my operating system on QEMU, I observed more frequent
> interruptions than I configured on the HPET timer. I investigated the cause
> and found that not only interruptions from HPET but also interruptions from
> i8254 were sent to IRQ2 because of a redirection from IRQ0 to IRQ2. This
> redirection is added in hw/apic.c at commit
> 16b29ae1807b024bd5052301550f5d47dae958a2 but this redirection caused wrong
> interruptions. So I deleted the redirection. Finally, I confirmed there is
> no problem on 'make check' results and that interruptions from i8254 and
> interruptions from HPET are correclty sent to IRQ0 and IRQ2 respectively.

Hi, did you set the legacy replacement route bit on the HPET?

My understanding is that:
- if you enable legacy-replacement routing, the HPET will take care of
dropping all i8254 interrupts
- if you disable legacy-replacement routing, the i8254 will still
generate interrupts on ISA IRQ 0. If you then enable the IO-APIC and
program the routes according to the ACPI MADT table, the interrupt
from the i8254 (ISA IRQ 0) will be redirected to the IO-APIC's GSI2.

So the solutions would be one of the following if you use HPET timer 0:
- disable the i8254
- enable legacy-replacement routing
- mask GSI2 on the IO-APIC and use a different route for the HPET
(worse, but should also work)

Thanks,

Paolo


> Signed-off-by: TaiseiIto <taisei1212@outlook.jp>
> ---
>  hw/intc/ioapic.c | 7 -------
>  1 file changed, 7 deletions(-)
>
> diff --git a/hw/intc/ioapic.c b/hw/intc/ioapic.c
> index 716ffc8bbb..6b630b45ca 100644
> --- a/hw/intc/ioapic.c
> +++ b/hw/intc/ioapic.c
> @@ -154,15 +154,8 @@ static void ioapic_set_irq(void *opaque, int vector, int 
> level)
>  {
>      IOAPICCommonState *s = opaque;
>
> -    /* ISA IRQs map to GSI 1-1 except for IRQ0 which maps
> -     * to GSI 2.  GSI maps to ioapic 1-1.  This is not
> -     * the cleanest way of doing it but it should work. */
> -
>      trace_ioapic_set_irq(vector, level);
>      ioapic_stat_update_irq(s, vector, level);
> -    if (vector == 0) {
> -        vector = 2;
> -    }
>      if (vector < IOAPIC_NUM_PINS) {
>          uint32_t mask = 1 << vector;
>          uint64_t entry = s->ioredtbl[vector];
> --
> 2.34.1
>