[patch V3 01/40] cpu/SMT: Make SMT control more robust against enumeration failures

From: Thomas Gleixner
Date: Wed Aug 02 2023 - 06:21:14 EST

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

This went unnoticed so far because by chance XEN/PV ends up with
smp_num_siblings == 2. So smt_hotplug_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 smt_hotplug_control is set to CPU_SMT_NOT_SUPPORTED
which causes cpu_smt_allowed() to evaluate the unpopulated primary thread
mask with the conclusion that all non-boot CPUs are not valid to be

Make cpu_smt_allowed() more robust and take CPU_SMT_NOT_SUPPORTED and

The primary mask issue on x86 XEN/PV needs to be addressed separately as
there are users outside of the CPU hotplug code too.

Fixes: 05736e4ac13c ("cpu/hotplug: Provide knobs to control SMT")
Reported-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
kernel/cpu.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -630,6 +630,12 @@ static inline bool cpu_smt_allowed(unsig
if (cpu_smt_control == CPU_SMT_ENABLED)
return true;

+ if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
+ return true;
+ if (cpu_smt_control == CPU_SMT_NOT_IMPLEMENTED)
+ return true;
if (topology_is_primary_thread(cpu))
return true;