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2230f9e117
We run into guest hang in edk2 firmware when KSM is kept as running on the host. The edk2 firmware is waiting for status 0x80 from QEMU's pflash device (TYPE_PFLASH_CFI01) during the operation of sector erasing or buffered write. The status is returned by reading the memory region of the pflash device and the read request should have been forwarded to QEMU and emulated by it. Unfortunately, the read request is covered by an illegal stage2 mapping when the guest hang issue occurs. The read request is completed with QEMU bypassed and wrong status is fetched. The edk2 firmware runs into an infinite loop with the wrong status. The illegal stage2 mapping is populated due to same page sharing by KSM at (C) even the associated memory slot has been marked as invalid at (B) when the memory slot is requested to be deleted. It's notable that the active and inactive memory slots can't be swapped when we're in the middle of kvm_mmu_notifier_change_pte() because kvm->mn_active_invalidate_count is elevated, and kvm_swap_active_memslots() will busy loop until it reaches to zero again. Besides, the swapping from the active to the inactive memory slots is also avoided by holding &kvm->srcu in __kvm_handle_hva_range(), corresponding to synchronize_srcu_expedited() in kvm_swap_active_memslots(). CPU-A CPU-B ----- ----- ioctl(kvm_fd, KVM_SET_USER_MEMORY_REGION) kvm_vm_ioctl_set_memory_region kvm_set_memory_region __kvm_set_memory_region kvm_set_memslot(kvm, old, NULL, KVM_MR_DELETE) kvm_invalidate_memslot kvm_copy_memslot kvm_replace_memslot kvm_swap_active_memslots (A) kvm_arch_flush_shadow_memslot (B) same page sharing by KSM kvm_mmu_notifier_invalidate_range_start : kvm_mmu_notifier_change_pte kvm_handle_hva_range __kvm_handle_hva_range kvm_set_spte_gfn (C) : kvm_mmu_notifier_invalidate_range_end Fix the issue by skipping the invalid memory slot at (C) to avoid the illegal stage2 mapping so that the read request for the pflash's status is forwarded to QEMU and emulated by it. In this way, the correct pflash's status can be returned from QEMU to break the infinite loop in the edk2 firmware. We tried a git-bisect and the first problematic commit is |
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async_pf.c | ||
async_pf.h | ||
binary_stats.c | ||
coalesced_mmio.c | ||
coalesced_mmio.h | ||
dirty_ring.c | ||
eventfd.c | ||
irqchip.c | ||
Kconfig | ||
kvm_main.c | ||
kvm_mm.h | ||
Makefile.kvm | ||
pfncache.c | ||
vfio.c | ||
vfio.h |