6.6-stable review patch. If anyone has any objections, please let me know.
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From: Thomas Gleixner tglx@linutronix.de
[ Upstream commit 965e05ff8af98c44f9937366715c512000373164 ]
The SMT control mechanism got added as speculation attack vector mitigation. The implemented logic relies on the primary thread mask to be set up properly.
This turns out to be an issue with XEN/PV guests because their CPU hotplug mechanics do not enumerate APICs and therefore the mask is never correctly populated.
This went unnoticed so far because by chance XEN/PV ends up with smp_num_siblings == 2. So cpu_smt_control stays at its default value CPU_SMT_ENABLED and the primary thread mask is never evaluated in the context of CPU hotplug.
This stopped "working" with the upcoming overhaul of the topology evaluation which legitimately provides a fake topology for XEN/PV. That sets smp_num_siblings to 1, which causes the core CPU hot-plug core to refuse to bring up the APs.
This happens because cpu_smt_control is set to CPU_SMT_NOT_SUPPORTED which causes cpu_bootable() to evaluate the unpopulated primary thread mask with the conclusion that all non-boot CPUs are not valid to be plugged.
The core code has already been made more robust against this kind of fail, but the primary thread mask really wants to be populated to avoid other issues all over the place.
Just fake the mask by pretending that all XEN/PV vCPUs are primary threads, which is consistent because all of XEN/PVs topology is fake or non-existent.
Fixes: 6a4d2657e048 ("x86/smp: Provide topology_is_primary_thread()") Fixes: f54d4434c281 ("x86/apic: Provide cpu_primary_thread mask") Reported-by: Juergen Gross jgross@suse.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Tested-by: Juergen Gross jgross@suse.com Tested-by: Sohil Mehta sohil.mehta@intel.com Tested-by: Michael Kelley mikelley@microsoft.com Tested-by: Peter Zijlstra (Intel) peterz@infradead.org Tested-by: Zhang Rui rui.zhang@intel.com Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lore.kernel.org/r/20230814085112.210011520@linutronix.de Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kernel/apic/apic.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c index 760adac3d1a82..3cdf48493546d 100644 --- a/arch/x86/kernel/apic/apic.c +++ b/arch/x86/kernel/apic/apic.c @@ -36,6 +36,8 @@ #include <linux/smp.h> #include <linux/mm.h>
+#include <xen/xen.h> + #include <asm/trace/irq_vectors.h> #include <asm/irq_remapping.h> #include <asm/pc-conf-reg.h> @@ -2344,6 +2346,15 @@ static int __init smp_init_primary_thread_mask(void) { unsigned int cpu;
+ /* + * XEN/PV provides either none or useless topology information. + * Pretend that all vCPUs are primary threads. + */ + if (xen_pv_domain()) { + cpumask_copy(&__cpu_primary_thread_mask, cpu_possible_mask); + return 0; + } + for (cpu = 0; cpu < nr_logical_cpuids; cpu++) cpu_mark_primary_thread(cpu, cpuid_to_apicid[cpu]); return 0;