[Xen-devel] Disabling lapic and apic: pitfalls?

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

Excuse my newbie-ness in this area... I hope the answers aren't
too obvious or widely known.

What are the disadvantages of disabling the LAPIC and APIC in a
Linux virtual machine?  What functionality is lost?  What class
of potential bugs might arise (or go away)?

Does the answer differ for hvm vs pvm?

Does the answer differ depending on whether the platform
underlying is Intel or AMD? 32-bit vs 64-bit?

Does the answer differ for vcpus=1 vs vcpus>1 (assuming CONFIG_SMP)

(And what about for Windows of various flavors?)

For example, I can see that if lapic is enabled, an extra 1000
interrupts per second are delivered to the VM (250 for pvm).
Are these useful for anything?  Or are they just eating VM cycles?

I suspect that disabled apic may break driver domains or pci
passthrough.  True?  What about lapic?

I ask all these questions because I often see problem responses of
the type "Try apic=0" or "Boot with noapic" and I wonder about the
side effects of such a cure.  Or whether the cure should be
universally applied for VMs: Depending on the answers to the
above questions, perhaps the default should be off and we
should be saying "Try apic=1 if you need that functionality".

Thanks,
Dan


===================================
If Xen could save time in a bottle / then clocks wouldn't virtually skew /
It would save every tick / for VMs that aren't quick /
and Xen then would send them anew
(with apologies to the late great Jim Croce)

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