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Re: [PATCH 1/1] x86: centralize default APIC id definition
- To: Jan Beulich <jbeulich@xxxxxxxx>, Alex Olson <this.is.a0lson@xxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 1 Oct 2021 15:29:45 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Alex Olson <alex.olson@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 01/10/2021 15:19, Jan Beulich wrote:
> On 24.09.2021 21:39, Alex Olson wrote:
>> Inspired by an earlier attempt by Chao Gao <chao.gao@xxxxxxxxx>,
>> this revision aims to put the hypervisor in control of x86 APIC identifier
>> definition instead of hard-coding a formula in multiple places
>> (libxl, hvmloader, hypervisor).
>>
>> This is intended as a first step toward exposing/altering CPU topology
>> seen by guests.
>>
>> Changes:
>>
>> - Add field to vlapic for holding default ID (on reset)
>>
>> - add HVMOP_get_vcpu_topology_id hypercall so libxl (for PVH domains)
>> can access APIC ids needed for ACPI table definition prior to domain start.
>>
>> - For HVM guests, hvmloader now also uses the same hypercall.
>>
>> - Make CPUID code use vlapic ID instead of hard-coded formula
>> for runtime reporting to guests
> I'm afraid a primary question from back at the time remains: How is
> migration of a guest from an old hypervisor to one with this change
> in place going to work?
I'm afraid its not.
Fixing this is incredibly complicated. I have a vague plan, but it
needs building on the still-pending libxl cpuid work of Rogers.
Both the toolstack and Xen need to learn about how to describe topology
correctly (and I'm afraid this patch isn't correct even for a number of
the simple cases), and know about "every VM booted up until this point
in time" being wrong.
~Andrew
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