Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there'sno local APIC

From: Jeremy Fitzhardinge
Date: Sun Jun 14 2009 - 22:07:03 EST


On 06/12/09 13:35, Eric W. Biederman wrote:
Jeremy Fitzhardinge<jeremy@xxxxxxxx> writes:

Parse the ACPI MADT for I/O APIC information, even if the cpu has no
(apparent) local APIC (ie, the CPU's APIC feature flag is clear).

In principle, the local APIC and the I/O APIC are distinct (but related)
components, which can be independently present.

In practice this can happen in a Xen system, where the hypervisor has
full control over the local APICs, and delivers interrupts initiated by
the I/O APICs via Xen's event channel mechanism.

Xen is giving us a semi bogus acpi table?

No, not really. The guest is reading the real BIOS-provided ACPI tables, but Xen is clobbering the APIC feature in CPUID so the virtual CPU doesn't appear to have a usable local APIC. Xen itself doesn't care very much about interrupt routing or ACPI, and doesn't make any attempt to read or parse the ACPI data itself (except for very basic things like the APIC addresses).
What is the paravirt configuration model with Xen? Is it documented
somewhere?

Not very well. The basic idea is that Xen owns the local apics, and does things like vector allocation. The guest kernel is responsible for asking for a vector, and doing the appropriate IO APIC programming, and binding that vector to an event channel. The interrupt is then delivered via the normal event channel mechanism already in place to deal with all the other event types an unprivileged domain can get.

J
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