diff options
| -rw-r--r-- | Documentation/virtual/kvm/mmu.txt | 14 | ||||
| -rw-r--r-- | arch/x86/kvm/mmu.c | 20 | ||||
| -rw-r--r-- | virt/kvm/kvm_main.c | 23 |
3 files changed, 42 insertions, 15 deletions
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt index 2908941..53838d9 100644 --- a/Documentation/virtual/kvm/mmu.txt +++ b/Documentation/virtual/kvm/mmu.txt @@ -425,6 +425,20 @@ fault through the slow path. Since only 19 bits are used to store generation-number on mmio spte, all pages are zapped when there is an overflow. +Unfortunately, a single memory access might access kvm_memslots(kvm) multiple +times, the last one happening when the generation number is retrieved and +stored into the MMIO spte. Thus, the MMIO spte might be created based on +out-of-date information, but with an up-to-date generation number. + +To avoid this, the generation number is incremented again after synchronize_srcu +returns; thus, the low bit of kvm_memslots(kvm)->generation is only 1 during a +memslot update, while some SRCU readers might be using the old copy. We do not +want to use an MMIO sptes created with an odd generation number, and we can do +this without losing a bit in the MMIO spte. The low bit of the generation +is not stored in MMIO spte, and presumed zero when it is extracted out of the +spte. If KVM is unlucky and creates an MMIO spte while the low bit is 1, +the next access to the spte will always be a cache miss. + Further reading =============== diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index 323c3f5..9651595 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -199,16 +199,20 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_mask) EXPORT_SYMBOL_GPL(kvm_mmu_set_mmio_spte_mask); /* - * spte bits of bit 3 ~ bit 11 are used as low 9 bits of generation number, - * the bits of bits 52 ~ bit 61 are used as high 10 bits of generation - * number. + * the low bit of the generation number is always presumed to be zero. + * This disables mmio caching during memslot updates. The concept is + * similar to a seqcount but instead of retrying the access we just punt + * and ignore the cache. + * + * spte bits 3-11 are used as bits 1-9 of the generation number, + * the bits 52-61 are used as bits 10-19 of the generation number. */ -#define MMIO_SPTE_GEN_LOW_SHIFT 3 +#define MMIO_SPTE_GEN_LOW_SHIFT 2 #define MMIO_SPTE_GEN_HIGH_SHIFT 52 -#define MMIO_GEN_SHIFT 19 -#define MMIO_GEN_LOW_SHIFT 9 -#define MMIO_GEN_LOW_MASK ((1 << MMIO_GEN_LOW_SHIFT) - 1) +#define MMIO_GEN_SHIFT 20 +#define MMIO_GEN_LOW_SHIFT 10 +#define MMIO_GEN_LOW_MASK ((1 << MMIO_GEN_LOW_SHIFT) - 2) #define MMIO_GEN_MASK ((1 << MMIO_GEN_SHIFT) - 1) #define MMIO_MAX_GEN ((1 << MMIO_GEN_SHIFT) - 1) @@ -4428,7 +4432,7 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm) * The very rare case: if the generation-number is round, * zap all shadow pages. */ - if (unlikely(kvm_current_mmio_generation(kvm) >= MMIO_MAX_GEN)) { + if (unlikely(kvm_current_mmio_generation(kvm) == 0)) { printk_ratelimited(KERN_INFO "kvm: zapping shadow pages for mmio generation wraparound\n"); kvm_mmu_invalidate_zap_all_pages(kvm); } diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 0bfdb67..bb8641b 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -95,8 +95,6 @@ static int hardware_enable_all(void); static void hardware_disable_all(void); static void kvm_io_bus_destroy(struct kvm_io_bus *bus); -static void update_memslots(struct kvm_memslots *slots, - struct kvm_memory_slot *new, u64 last_generation); static void kvm_release_pfn_dirty(pfn_t pfn); static void mark_page_dirty_in_slot(struct kvm *kvm, @@ -695,8 +693,7 @@ static void sort_memslots(struct kvm_memslots *slots) } static void update_memslots(struct kvm_memslots *slots, - struct kvm_memory_slot *new, - u64 last_generation) + struct kvm_memory_slot *new) { if (new) { int id = new->id; @@ -707,8 +704,6 @@ static void update_memslots(struct kvm_memslots *slots, if (new->npages != npages) sort_memslots(slots); } - - slots->generation = last_generation + 1; } static int check_memory_region_flags(struct kvm_userspace_memory_region *mem) @@ -730,10 +725,24 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, { struct kvm_memslots *old_memslots = kvm->memslots; - update_memslots(slots, new, kvm->memslots->generation); + /* + * Set the low bit in the generation, which disables SPTE caching + * until the end of synchronize_srcu_expedited. + */ + WARN_ON(old_memslots->generation & 1); + slots->generation = old_memslots->generation + 1; + + update_memslots(slots, new); rcu_assign_pointer(kvm->memslots, slots); synchronize_srcu_expedited(&kvm->srcu); + /* + * Increment the new memslot generation a second time. This prevents + * vm exits that race with memslot updates from caching a memslot + * generation that will (potentially) be valid forever. + */ + slots->generation++; + kvm_arch_memslots_updated(kvm); return old_memslots; |
