On Wed, Jan 12, 2022 at 05:35:12PM +0000, Sean Christopherson wrote: >On Wed, Jan 12, 2022, Chao Gao wrote: >> On Tue, Jan 11, 2022 at 07:48:39PM +0000, Sean Christopherson wrote: >> >On Tue, Jan 11, 2022, Tian, Kevin wrote: >> >> > From: Sean Christopherson <seanjc@xxxxxxxxxx> >> >> > Sent: Tuesday, January 11, 2022 7:00 AM >> >> > >> >> > On Mon, Dec 27, 2021, Chao Gao wrote: >> >> > > kvm_arch_check_processor_compat() needn't be called with interrupt >> >> > > disabled, as it only reads some CRs/MSRs which won't be clobbered >> >> > > by interrupt handlers or softirq. >> >> > > >> >> > > What really needed is disabling preemption. No additional check is >> >> > > added because if CONFIG_DEBUG_PREEMPT is enabled, smp_processor_id() >> >> > > (right above the WARN_ON()) can help to detect any violation. >> >> > >> >> > Hrm, IIRC, the assertion that IRQs are disabled was more about detecting >> >> > improper usage with respect to KVM doing hardware enabling than it was >> >> > about ensuring the current task isn't migrated. E.g. as exhibited by patch >> >> > 06, extra protections (disabling of hotplug in that case) are needed if >> >> > this helper is called outside of the core KVM hardware enabling flow since >> >> > hardware_enable_all() does its thing via SMP function call. >> >> >> >> Looks the WARN_ON() was added by you. 😊 >> > >> >Yeah, past me owes current me a beer. >> > >> >> commit f1cdecf5807b1a91829a2dc4f254bfe6bafd4776 >> >> Author: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> >> >> Date: Tue Dec 10 14:44:14 2019 -0800 >> >> >> >> KVM: x86: Ensure all logical CPUs have consistent reserved cr4 bits >> >> >> >> Check the current CPU's reserved cr4 bits against the mask calculated >> >> for the boot CPU to ensure consistent behavior across all CPUs. >> >> >> >> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> >> >> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> >> >> >> >> But it's unclear to me how this WARN_ON() is related to what the commit >> >> msg tries to explain. >> > >> >Ya, the changelog and lack of a comment is awful. >> > >> >> When I read this code it's more like a sanity check on the assumption that it >> >> is currently called in SMP function call which runs the said function with >> >> interrupt disabled. >> > >> >Yes, and as above, that assertion was more about the helper not really being safe >> >for general usage as opposed to wanting to detect use from preemptible context. >> >If we end up keeping the WARN_ON, I'll happily write a comment explaining the >> >point of the assertion. >> >> OK. I will do following changes to keep the WARN_ON(): >> 1. drop this patch >> 2. disable interrupt before the call site in patch 6. > >No, we shouldn't sully other code just to keep this WARN. Again, the point of >the WARN is/was to highlight that any use outside of the hardware enabling path >is suspect. That's why I asked if there was a way this code could identify that >the CPU in question is being hotplugged, i.e. to convey that the helper is safe >to use only during hardware enabling _or_ hotplug. If that's not feasible, >replacing the WARN with a scary comment is better than disabling IRQs. OK. How about: /* * Compatibility checks are done when loading KVM or in KVM's CPU * hotplug callback. It ensures all online CPUs are compatible before * running any vCPUs. For other cases, compatibility checks are * unnecessary or even problematic. Try to detect improper usages here. */ WARN_ON(!irqs_disabled() && !cpu_active(smp_processor_id())); a CPU is active when it reaches the CPUHP_AP_ACTIVE state (the last state before CPUHP_ONLINE). So, if a cpu isn't active, it probably is being hotplugged. One false positive is the CPU is dying, which I guess is fine. And to help justify this change, I will merge it into patch 6.
