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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

APIC is required for multiprocessor HVM guests. It can be lived without for
UP guests.

 -- Keir

On 14/3/08 17:55, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> 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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