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 17:04, Sergio Lopez wrote:
>> I'm going back to this level of the thread, because after your
>> suggestion I took a deeper look at how things work around the PIC, and
>> discovered I was completely wrong about my assumptions.
>> 
>> For virtio-mmio devices, given that we don't have the ability to
>> configure vectors (as it's done in the PCI case) we're stuck with the
>> ones provided by the platform PIC, which in the x86 case is the i8259
>> (at least from Linux's perspective).
>> 
>> So we can get rid of the IOAPIC, but we need to keep the i8259 (we have
>> both a userspace and a kernel implementation too, so it should be fine).
>
> Hmm...  I would have thought the vectors are just GSIs, which will be
> configured to the IOAPIC if it is present.  Maybe something is causing
> Linux to ignore the IOAPIC?

Turns out it was a bug in microvm. I was writing 0 to FW_CFG_NB_CPUS
(because I was using x86ms->boot_cpus instead of ms->smp.cpus), which
led to a broken MP table, causing Linux to ignore it and, as a side
effect to disable IOAPIC symmetric I/O mode.

After fixing it we can, indeed, boot without the i8259 \o/ :

/ # dmesg | grep legacy
[    0.074144] Using NULL legacy PIC
/ # cat /pr[   12.116930] random: fast init done
/ # cat /proc/interrupts 
           CPU0       CPU1       
  4:          0        278   IO-APIC   4-edge      ttyS0
 12:         48          0   IO-APIC  12-edge      virtio0
NMI:          0          0   Non-maskable interrupts
LOC:        124         98   Local timer interrupts
SPU:          0          0   Spurious interrupts
PMI:          0          0   Performance monitoring interrupts
IWI:          0          0   IRQ work interrupts
RTR:          0          0   APIC ICR read retries
RES:        476        535   Rescheduling interrupts
CAL:          0         76   Function call interrupts
TLB:          0          0   TLB shootdowns
HYP:          0          0   Hypervisor callback interrupts
ERR:          0
MIS:          0
PIN:          0          0   Posted-interrupt notification event
NPI:          0          0   Nested posted-interrupt event
PIW:          0          0   Posted-interrupt wakeup event

There's still one problem. If the Guest doesn't have TSC_DEADLINE_TIME,
Linux hangs on APIC timer calibration. I'm looking for a way to work
around this. Worst case scenario, we can check for that feature and add
both PIC and PIT if is missing.

>> As for the PIT, we can omit it if we're running with KVM acceleration,
>> as kvmclock will be used to calculate loops per jiffie and avoid the
>> calibration, leaving it enabled otherwise.
>
> Can you make it an OnOffAuto property, and default to on iff !KVM?

Sure.

Thanks,
Sergio.

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