Re: [PATCH 1/1] x86: SMT broken on Xen PV DomU since 6.9 (updated)

[Niels Dettenbach <nd@syndicat.com>]

Am Mittwoch, 11. September 2024, 10:53:30  schrieb Niels Dettenbach:
> 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 assuming a "crash kernel" and disable
> SMT (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 SMT / all CPUs as in the past to such DomU
> VMs.
> 
> Signed-off-by: Niels Dettenbach <nd@syndicat.com>
> 

Signed-off-by: Niels Dettenbach <nd@syndicat.com>
---

A reworked proposal patch which substitutes my initial proposed patch:


--- linux/arch/x86/kernel/cpu/topology.c        2024-09-11 17:42:42.699278317 +0200
+++ linux/arch/x86/kernel/cpu/topology.c.orig   2024-09-11 09:53:16.194095250 +0200
@@ -132,14 +132,6 @@
        u64 msr;

        /*
-        * assume Xen PV has a working (special) topology
-        */
-       if (xen_pv_domain()) {
-               topo_info.real_bsp_apic_id = topo_info.boot_cpu_apic_id;
-               return false;
-       }
-
-       /*
         * There is no real good way to detect whether this a kdump()
         * kernel, but except on the Voyager SMP monstrosity which is not
         * longer supported, the real BSP APIC ID is the first one which is