Paolo Bonzini <pbonz...@redhat.com> writes: > On 25/09/19 10:40, Sergio Lopez wrote: >>>> We need the PIT for non-KVM accel (if present with KVM and >>>> kernel_irqchip_split = off, it basically becomes a placeholder) >>> Why? >> >> Perhaps I'm missing something. Is some other device supposed to be >> acting as a HW timer while running with TCG acceleration? > > Sure, the LAPIC timer. I wonder if Linux, however, wants to use the PIT > in order to calibrate the LAPIC timer if TSC deadline mode is unavailable.
Ah, yes. I was so confused by the nomenclature that I assumed we didn't have a userspace implementation of it. On the other hand, as you suspect, without the PIT Linux does hang in TSC calibration with TCG accel. A simple option could be adding it only if we're running without KVM. >>>> and the PIC for both the PIT and the ISA serial port. >>> >>> Can't the ISA serial port work with the IOAPIC? >> >> Hm... I'm not sure. I wanted to give it a try, but then noticed that >> multiple places in the code (like hw/intc/apic.c:560) do expect to have >> an ISA PIC present through the isa_pic global variable. >> >> I guess we should be able to work around this, but I'm not sure if it's >> really worth it. What do you think? > > You can add a paragraph saying that in the future the list could be > reduced further. I think that the direction we want to go is to only > leave the IOAPIC around (the ISA devices in this respect are no > different from the virtio-mmio devices). > > But you're right about isa_pic. I wonder if it's as easy as this: > > diff --git a/hw/intc/apic.c b/hw/intc/apic.c > index bce89911dc..5d03e48a19 100644 > --- a/hw/intc/apic.c > +++ b/hw/intc/apic.c > @@ -610,7 +610,7 @@ int apic_accept_pic_intr(DeviceState *dev) > > if ((s->apicbase & MSR_IA32_APICBASE_ENABLE) == 0 || > (lvt0 & APIC_LVT_MASKED) == 0) > - return 1; > + return isa_pic != NULL; > > return 0; > } Yes, that would do the trick. There's another use of it at hw/intc/ioapic.c:78, but we should be safe as, at least in the case of Linux, DM_EXTINT is only used in check_timer(), which is only called if it detects a i8259 PIC. We should probably add an assertion with an informative message, just in case. Thanks, Sergio.
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