On Mon, Jan 17, 2022 at 09:35:04PM +0800, Chao Gao wrote: >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())); Sorry. It should be: 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.
