On 04/03/2016 08:12, Lan Tianyu wrote: > > > /* > > > - * wmb: make sure everyone sees our modifications to the page tables > > > - * rmb: make sure we see changes to vcpu->mode > > > > You want to leave the comment explaining the memory barriers and tell that > > kvm_flush_remote_tlbs() contains the smp_mb(). > > That sounds more reasonable. Will update. Thanks. In fact, the reason for kvm_flush_remote_tlbs()'s barrier is exactly what was in this comment. So you can: 1) add a comment to kvm_flush_remote_tlbs like: /* * We want to publish modifications to the page tables before reading * mode. Pairs with a memory barrier in arch-specific code. * - x86: smp_mb__after_srcu_read_unlock in vcpu_enter_guest. * - powerpc: smp_mb in kvmppc_prepare_to_enter. */ 2) add a comment to vcpu_enter_guest and kvmppc_prepare_to_enter, saying that the memory barrier also orders the write to mode from any reads to the page tables done while the VCPU is running. In other words, on entry a single memory barrier achieves two purposes (write ->mode before reading requests, write ->mode before reading page tables). The same should be true in kvm_flush_remote_tlbs(). So you may investigate removing the barrier from kvm_flush_remote_tlbs, because kvm_make_all_cpus_request already includes a memory barrier. Like Thomas suggested, leave a comment in kvm_flush_remote_tlbs(), saying which memory barrier you are relying on and for what. And finally, the memory barrier in kvm_make_all_cpus_request can become smp_mb__after_atomic, which is free on x86. Of course, all this should be done in at least three separate patches. Thanks! Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
