On Thu, Sep 12, 2024 at 10:48:14AM +0200, Niels Dettenbach wrote:
Virtual machines under Xen Hypervisor (DomU) running in Xen PV mode use a special, nonstandard synthetized CPU topology which "just works" under kernels 6.9.x while newer kernels wrongly assuming a "crash kernel" and disable SMP (reducing to one CPU core) because the newer topology implementation produces a wrong error "[Firmware Bug]: APIC enumeration order not specification compliant" after new topology checks which are improper for Xen PV platform. As a result, the kernel disables SMT and activates just one CPU core within the VM (DomU).
The patch disables the regarding checks if it is running in Xen PV mode (only) and bring back SMP / all CPUs as in the past to such DomU VMs.
Signed-off-by: Niels Dettenbach nd@syndicat.com
The current behaviour leads all of our production Xen Host platforms (amd64 - HPE proliant) unusable after updating to newer linux kernels (with just one CPU available/activated per VM) while older kernels and other OS (current NetBSD PV DomU) still work fully (and stable since many years on the platform).
Xen PV mode is still provided by current Xen and widely used - even if less wide as the newer Xen PVH mode today. So a solution probably will be required.
So we assume that bug affects stable@vger.kernel.org as well.
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This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.
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