Re: [Qemu-devel] [PATCH v2] pc: Fix and clean up PIC-to-APIC IRQ path

[Blue Swirl]

On Sat, Sep 3, 2011 at 11:17 AM, Jan Kiszka <address@hidden> wrote:
> On 2011-09-03 10:58, Blue Swirl wrote:
>> On Thu, Sep 1, 2011 at 9:08 AM, Jan Kiszka <address@hidden> wrote:
>>> The master PIC is connected to the LINTIN0 of the APICs. As the APIC
>>> currently does not track the state of that line, we have to ask the PIC
>>> to reinject its IRQ after the CPU picked up an event from the APIC.
>>>
>>> This introduces pic_get_output to read the master PIC IRQ line state
>>> without changing it. The APIC uses this function to decide if a PIC IRQ
>>> should be reinjected on apic_update_irq. This reflects better how the
>>> real hardware works.
>>>
>>> The patch fixes some failures of the kvm unit tests apic and eventinj by
>>> allowing to enable the proper CPU IRQ deassertion when the guest masks
>>> some pending IRQs at PIC level.
>>>
>>> Signed-off-by: Jan Kiszka <address@hidden>
>>> ---
>>>
>>> Changes in v2:
>>>  - Avoid adding pic_level to the APIC state, obtain PIC output via
>>>   pic_get_level instead
>>>  - Do not reassert PIC interrupt if APIC is not accepting it
>>>  - Use apic_deliver_pic_intr for reassertion to ensure correct
>>>   processing
>>>
>>> This is not as nice as the previous version /wrt the interaction of PIC
>>> and APIC. But it avoids breaking the APIC vmstate for the sake of
>>> internal changes, also keeping it compatible with the upcoming KVM
>>> in-kernel APIC (that allows no easy pic_level state extraction). The
>>> interconnection between PIC and APIC may look nicer in the future with
>>> QOM. And in the end this just reflects the "beauty" of the x86
>>> architecture.
>>>
>>>  hw/apic.c  |    4 ++++
>>>  hw/i8259.c |   15 +++++++--------
>>>  hw/pc.c    |    3 ---
>>>  hw/pc.h    |    2 +-
>>>  4 files changed, 12 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/hw/apic.c b/hw/apic.c
>>> index d8f56c8..8289eef 100644
>>> --- a/hw/apic.c
>>> +++ b/hw/apic.c
>>> @@ -23,6 +23,7 @@
>>>  #include "host-utils.h"
>>>  #include "sysbus.h"
>>>  #include "trace.h"
>>> +#include "pc.h"
>>>
>>>  /* APIC Local Vector Table */
>>>  #define APIC_LVT_TIMER   0
>>> @@ -399,6 +400,9 @@ static void apic_update_irq(APICState *s)
>>>     }
>>>     if (apic_irq_pending(s) > 0) {
>>>         cpu_interrupt(s->cpu_env, CPU_INTERRUPT_HARD);
>>> +    } else if (apic_accept_pic_intr(&s->busdev.qdev) &&
>>> +               pic_get_output(isa_pic)) {
>>
>> This is indeed ugly. Why doesn't APIC track PIC output?
>
> For the reasons explained above.

Breaking vmstate compatibility? Don't we have all kinds of
compatibility stuff for this?

I'm not opposed to the patch as is, it's still a cleanup, but I wish
this ugliness could be avoided.