Re: [PATCH v4 7/8] docs/microvm.txt: document the new microvm machine type

[Paolo Bonzini]

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.

>>> 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;
 }

Thanks,

Paolo

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