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.
