From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>, Alex Olson <this.is.a0lson@gmail.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
"Ian Jackson" <iwj@xenproject.org>,
"George Dunlap" <george.dunlap@citrix.com>,
"Julien Grall" <julien@xen.org>,
"Stefano Stabellini" <sstabellini@kernel.org>,
"Juergen Gross" <jgross@suse.com>,
"Anthony PERARD" <anthony.perard@citrix.com>,
"Alex Olson" <alex.olson@starlab.io>,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH 1/1] x86: centralize default APIC id definition
Date: Fri, 1 Oct 2021 15:29:45 +0100 [thread overview]
Message-ID: <d63a0e7d-7f9d-0dcb-0ac9-8995e56698b4@citrix.com> (raw)
In-Reply-To: <d899b058-a201-e2f8-35d2-f0e59ab4bab3@suse.com>
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@intel.com>,
>> 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
next prev parent reply other threads:[~2021-10-01 14:31 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-24 19:39 [PATCH 0/1] x86: centralize default APIC id definition Alex Olson
2021-09-24 19:39 ` [PATCH 1/1] " Alex Olson
2021-10-01 14:19 ` Jan Beulich
2021-10-01 14:29 ` Andrew Cooper [this message]
2021-10-01 15:08 ` Juergen Gross
2021-10-01 17:38 ` Andrew Cooper
2021-10-01 19:07 ` Alex Olson
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