KVM: selftests: Sync data verify of dirty logging with guest sync
This fixes a bug that can trigger with e.g. "taskset -c 0 ./dirty_log_test" or when the testing host is very busy. A similar previous attempt is done [1] but that is not enough, the reason is stated in the reply [2]. As a summary (partly quotting from [2]): The problem is I think one guest memory write operation (of this specific test) contains a few micro-steps when page is during kvm dirty tracking (here I'm only considering write-protect rather than pml but pml should be similar at least when the log buffer is full): (1) Guest read 'iteration' number into register, prepare to write, page fault (2) Set dirty bit in either dirty bitmap or dirty ring (3) Return to guest, data written When we verify the data, we assumed that all these steps are "atomic", say, when (1) happened for this page, we assume (2) & (3) must have happened. We had some trick to workaround "un-atomicity" of above three steps, as previous version of this patch wanted to fix atomicity of step (2)+(3) by explicitly letting the main thread wait for at least one vmenter of vcpu thread, which should work. However what I overlooked is probably that we still have race when (1) and (2) can be interrupted. One example calltrace when it could happen that we read an old interation, got interrupted before even setting the dirty bit and flushing data: __schedule+1742 __cond_resched+52 __get_user_pages+530 get_user_pages_unlocked+197 hva_to_pfn+206 try_async_pf+132 direct_page_fault+320 kvm_mmu_page_fault+103 vmx_handle_exit+288 vcpu_enter_guest+2460 kvm_arch_vcpu_ioctl_run+325 kvm_vcpu_ioctl+526 __x64_sys_ioctl+131 do_syscall_64+51 entry_SYSCALL_64_after_hwframe+68 It means iteration number cached in vcpu register can be very old when dirty bit set and data flushed. So far I don't see an easy way to guarantee all steps 1-3 atomicity but to sync at the GUEST_SYNC() point of guest code when we do verification of the dirty bits as what this patch does. [1] https://lore.kernel.org/lkml/20210413213641.23742-1-peterx@redhat.com/ [2] https://lore.kernel.org/lkml/20210417140956.GV4440@xz-x1/ Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Sean Christopherson <seanjc@google.com> Cc: Andrew Jones <drjones@redhat.com> Cc: stable@vger.kernel.org Signed-off-by:Peter Xu <peterx@redhat.com> Message-Id: <20210417143602.215059-2-peterx@redhat.com> Signed-off-by:
Paolo Bonzini <pbonzini@redhat.com>
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