On 24/05/18 12:39, Will Deacon wrote:
On Thu, May 24, 2018 at 10:58:43AM +0100, Suzuki K Poulose wrote:
On 22/05/18 16:06, Marc Zyngier wrote:
As for Spectre variant-2, we rely on SMCCC 1.1 to provide the
discovery mechanism for detecting the SSBD mitigation.
A new capability is also allocated for that purpose, and a
config option.
Signed-off-by: Marc Zyngier <marc.zyng...@arm.com>
+static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
+ int scope)
+{
+ struct arm_smccc_res res;
+ bool supported = true;
+
+ WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
+
+ if (psci_ops.smccc_version == SMCCC_VERSION_1_0)
+ return false;
+
+ /*
+ * The probe function return value is either negative
+ * (unsupported or mitigated), positive (unaffected), or zero
+ * (requires mitigation). We only need to do anything in the
+ * last case.
+ */
+ switch (psci_ops.conduit) {
+ case PSCI_CONDUIT_HVC:
+ arm_smccc_1_1_hvc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
+ ARM_SMCCC_ARCH_WORKAROUND_2, &res);
+ if ((int)res.a0 != 0)
+ supported = false;
+ break;
+
+ case PSCI_CONDUIT_SMC:
+ arm_smccc_1_1_smc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
+ ARM_SMCCC_ARCH_WORKAROUND_2, &res);
+ if ((int)res.a0 != 0)
+ supported = false;
+ break;
+
+ default:
+ supported = false;
+ }
+
+ if (supported) {
+ __this_cpu_write(arm64_ssbd_callback_required, 1);
+ do_ssbd(true);
+ }
Marc,
As discussed, we have minor issue with the "corner case". If a CPU
is hotplugged in which requires the mitigation, after the system has
finalised the cap to "not available", the CPU could go ahead and
do the "work around" as above, while not effectively doing anything
about it at runtime for KVM guests (as thats the only place where
we rely on the CAP being set).
But, yes this is real corner case. There is no easy way to solve it
other than
1) Allow late modifications to CPU hwcaps
OR
2) Penalise the fastpath to always check per-cpu setting.
Shouldn't we just avoid bring up CPUs that require the mitigation after
we've finalised the capability to say that it's not required? Assuming this
is just another issue with maxcpus=, then I don't much care for it.
Ah! Sorry, yes we do kill the CPU. But it is just that it will set the
ssbd_callback_required flag and issue the do_ssbd(), which is not an issue.
Yes this can only be triggered by maxcpus=.
Suzuki
