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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 14/3/08 19:06, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> APIC is required for multiprocessor HVM guests. It can be
>> lived without for UP guests.
> 
> LAPIC or (IO)APIC or both?  And a key part of the question
> is what does "lived without" mean (as far as functionality
> or performance or stability or ??? lost)?

Without LAPIC the IO-APIC is unusable, since IO-APIC-routed interrupts can
only be received by a LAPIC. The advantage of an IO-APIC beyond legacy PIC
is mainly that there are more PCI interrupt pins -- on our emulated IO-APIC
there are 32 of them, as opposed to just four PCI-ISA links. This means that
an IO-APIC is much less likely to introduce interrupt sharing among PCI
devices. However, there are few enough PCI devices emulated for an HVM guest
that the risk of interrupt sharing is probably not too concerning.

Beyond that, the LAPIC doesn't provide much of value in a uniprocessor
environment. It has its own timer, but there are other sources for that
(PIT, HPET, ...). Most other stuff is only applicable to MP systems.

 -- Keir



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