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.
