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

[Sergio Lopez <slp@redhat.com>]

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Paolo Bonzini <pbonzini@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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