6.1-stable review patch. If anyone has any objections, please let me know.
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From: Thomas Gleixner tglx@linutronix.de
[ Upstream commit d91bdd96b55cc3ce98d883a60f133713821b80a6 ]
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 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 plugged.
Make cpu_smt_allowed() more robust and take CPU_SMT_NOT_SUPPORTED and CPU_SMT_NOT_IMPLEMENTED into account. Rename it to cpu_bootable() while at it as that makes it more clear what the function is about.
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@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/20230814085112.149440843@linutronix.de Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/cpu.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/kernel/cpu.c b/kernel/cpu.c index c37f1758a4865..e6f0101941ed8 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -460,11 +460,19 @@ static inline bool cpu_smt_thread_allowed(unsigned int cpu) #endif }
-static inline bool cpu_smt_allowed(unsigned int cpu) +static inline bool cpu_bootable(unsigned int cpu) { if (cpu_smt_control == CPU_SMT_ENABLED && cpu_smt_thread_allowed(cpu)) return true;
+ /* All CPUs are bootable if controls are not configured */ + if (cpu_smt_control == CPU_SMT_NOT_IMPLEMENTED) + return true; + + /* All CPUs are bootable if CPU is not SMT capable */ + if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED) + return true; + if (topology_is_primary_thread(cpu)) return true;
@@ -485,7 +493,7 @@ bool cpu_smt_possible(void) } EXPORT_SYMBOL_GPL(cpu_smt_possible); #else -static inline bool cpu_smt_allowed(unsigned int cpu) { return true; } +static inline bool cpu_bootable(unsigned int cpu) { return true; } #endif
static inline enum cpuhp_state @@ -588,10 +596,10 @@ static int bringup_wait_for_ap(unsigned int cpu) * SMT soft disabling on X86 requires to bring the CPU out of the * BIOS 'wait for SIPI' state in order to set the CR4.MCE bit. The * CPU marked itself as booted_once in notify_cpu_starting() so the - * cpu_smt_allowed() check will now return false if this is not the + * cpu_bootable() check will now return false if this is not the * primary sibling. */ - if (!cpu_smt_allowed(cpu)) + if (!cpu_bootable(cpu)) return -ECANCELED;
if (st->target <= CPUHP_AP_ONLINE_IDLE) @@ -1478,7 +1486,7 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target) err = -EBUSY; goto out; } - if (!cpu_smt_allowed(cpu)) { + if (!cpu_bootable(cpu)) { err = -EPERM; goto out; }