From: Sean Christopherson Date: Fri, 16 May 2025 21:35:38 +0000 (-0700) Subject: KVM: Check for empty mask of harvested dirty ring entries in caller X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ee188dea1677a0d9b3a8097e71b803896b8aaed7;p=thirdparty%2Flinux.git KVM: Check for empty mask of harvested dirty ring entries in caller When resetting a dirty ring, explicitly check that there is work to be done before calling kvm_reset_dirty_gfn(), e.g. if no harvested entries are found and/or on the loop's first iteration, and delete the extremely misleading comment "This is only needed to make compilers happy". KVM absolutely relies on mask to be zero-initialized, i.e. the comment is an outright lie. Furthermore, the compiler is right to complain that KVM is calling a function with uninitialized data, as there are no guarantees the implementation details of kvm_reset_dirty_gfn() will be visible to kvm_dirty_ring_reset(). While the flaw could be fixed by simply deleting (or rewording) the comment, and duplicating the check is unfortunate, checking mask in the caller will allow for additional cleanups. Opportunistically drop the zero-initialization of cur_slot and cur_offset. If a bug were introduced where either the slot or offset was consumed before mask is set to a non-zero value, then it is highly desirable for the compiler (or some other sanitizer) to yell. Cc: Peter Xu Cc: Yan Zhao Cc: Maxim Levitsky Cc: Binbin Wu Reviewed-by: Pankaj Gupta Reviewed-by: James Houghton Reviewed-by: Binbin Wu Reviewed-by: Yan Zhao Reviewed-by: Peter Xu Link: https://lore.kernel.org/r/20250516213540.2546077-5-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c index 97cca0c02fd17..939198ac66954 100644 --- a/virt/kvm/dirty_ring.c +++ b/virt/kvm/dirty_ring.c @@ -55,9 +55,6 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask) struct kvm_memory_slot *memslot; int as_id, id; - if (!mask) - return; - as_id = slot >> 16; id = (u16)slot; @@ -108,15 +105,24 @@ static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn) int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring, int *nr_entries_reset) { + /* + * To minimize mmu_lock contention, batch resets for harvested entries + * whose gfns are in the same slot, and are within N frame numbers of + * each other, where N is the number of bits in an unsigned long. For + * simplicity, process the current set of entries when the next entry + * can't be included in the batch. + * + * Track the current batch slot, the gfn offset into the slot for the + * batch, and the bitmask of gfns that need to be reset (relative to + * offset). Note, the offset may be adjusted backwards, e.g. so that + * a sequence of gfns X, X-1, ... X-N-1 can be batched. + */ u32 cur_slot, next_slot; u64 cur_offset, next_offset; - unsigned long mask; + unsigned long mask = 0; struct kvm_dirty_gfn *entry; bool first_round = true; - /* This is only needed to make compilers happy */ - cur_slot = cur_offset = mask = 0; - while (likely((*nr_entries_reset) < INT_MAX)) { if (signal_pending(current)) return -EINTR; @@ -164,14 +170,34 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring, continue; } } - kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask); + + /* + * Reset the slot for all the harvested entries that have been + * gathered, but not yet fully processed. + */ + if (mask) + kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask); + + /* + * The current slot was reset or this is the first harvested + * entry, (re)initialize the metadata. + */ cur_slot = next_slot; cur_offset = next_offset; mask = 1; first_round = false; } - kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask); + /* + * Perform a final reset if there are harvested entries that haven't + * been processed, which is guaranteed if at least one harvested was + * found. The loop only performs a reset when the "next" entry can't + * be batched with the "current" entry(s), and that reset processes the + * _current_ entry(s); i.e. the last harvested entry, a.k.a. next, will + * always be left pending. + */ + if (mask) + kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask); /* * The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared