Re: [PATCH 1/1] x86: SMP broken on Xen PV DomU since 6.9

[Greg KH <gregkh@linuxfoundation.org>]

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.
> 

<formletter>

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.

</formletter>