diff options
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/kvm/mmu/mmu.c | 42 |
1 files changed, 35 insertions, 7 deletions
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 091278a8bd56..8e477333a263 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2920,6 +2920,31 @@ static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep) __direct_pte_prefetch(vcpu, sp, sptep); } +/* + * Lookup the mapping level for @gfn in the current mm. + * + * WARNING! Use of host_pfn_mapping_level() requires the caller and the end + * consumer to be tied into KVM's handlers for MMU notifier events! + * + * There are several ways to safely use this helper: + * + * - Check mmu_notifier_retry_hva() after grabbing the mapping level, before + * consuming it. In this case, mmu_lock doesn't need to be held during the + * lookup, but it does need to be held while checking the MMU notifier. + * + * - Hold mmu_lock AND ensure there is no in-progress MMU notifier invalidation + * event for the hva. This can be done by explicit checking the MMU notifier + * or by ensuring that KVM already has a valid mapping that covers the hva. + * + * - Do not use the result to install new mappings, e.g. use the host mapping + * level only to decide whether or not to zap an entry. In this case, it's + * not required to hold mmu_lock (though it's highly likely the caller will + * want to hold mmu_lock anyways, e.g. to modify SPTEs). + * + * Note! The lookup can still race with modifications to host page tables, but + * the above "rules" ensure KVM will not _consume_ the result of the walk if a + * race with the primary MMU occurs. + */ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, const struct kvm_memory_slot *slot) { @@ -2942,16 +2967,19 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, hva = __gfn_to_hva_memslot(slot, gfn); /* - * Lookup the mapping level in the current mm. The information - * may become stale soon, but it is safe to use as long as - * 1) mmu_notifier_retry was checked after taking mmu_lock, and - * 2) mmu_lock is taken now. - * - * We still need to disable IRQs to prevent concurrent tear down - * of page tables. + * Disable IRQs to prevent concurrent tear down of host page tables, + * e.g. if the primary MMU promotes a P*D to a huge page and then frees + * the original page table. */ local_irq_save(flags); + /* + * Read each entry once. As above, a non-leaf entry can be promoted to + * a huge page _during_ this walk. Re-reading the entry could send the + * walk into the weeks, e.g. p*d_large() returns false (sees the old + * value) and then p*d_offset() walks into the target huge page instead + * of the old page table (sees the new value). + */ pgd = READ_ONCE(*pgd_offset(kvm->mm, hva)); if (pgd_none(pgd)) goto out; |