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Subject[PATCH v4 17/19] KVM: Terminate memslot walks via used_slots
Date
Refactor memslot handling to treat the number of used slots as the de
facto size of the memslot array, e.g. return NULL from id_to_memslot()
when an invalid index is provided instead of relying on npages==0 to
detect an invalid memslot. Rework the sorting and walking of memslots
in advance of dynamically sizing memslots to aid bisection and debug,
e.g. with luck, a bug in the refactoring will bisect here and/or hit a
WARN instead of randomly corrupting memory.

Alternatively, a global null/invalid memslot could be returned, i.e. so
callers of id_to_memslot() don't have to explicitly check for a NULL
memslot, but that approach runs the risk of introducing difficult-to-
debug issues, e.g. if the global null slot is modified. Constifying
the return from id_to_memslot() to combat such issues is possible, but
would require a massive refactoring of arch specific code and would
still be susceptible to casting shenanigans.

Add function comments to update_memslots() and search_memslots() to
explicitly (and loudly) state how memslots are sorted.

No functional change intended.

Tested-by: Christoffer Dall <christoffer.dall@arm.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
arch/powerpc/kvm/book3s_hv.c | 2 +-
arch/x86/kvm/x86.c | 14 +--
include/linux/kvm_host.h | 18 ++-
virt/kvm/arm/mmu.c | 9 +-
virt/kvm/kvm_main.c | 220 ++++++++++++++++++++++++++---------
5 files changed, 189 insertions(+), 74 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 04d5b7cf874f..c15cabb58e09 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -4410,7 +4410,7 @@ static int kvm_vm_ioctl_get_dirty_log_hv(struct kvm *kvm,
slots = kvm_memslots(kvm);
memslot = id_to_memslot(slots, log->slot);
r = -ENOENT;
- if (!memslot->dirty_bitmap)
+ if (!memslot || !memslot->dirty_bitmap)
goto out;

/*
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 87fca25d5217..39ba1de85575 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9532,9 +9532,9 @@ void kvm_arch_sync_events(struct kvm *kvm)
int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
{
int i, r;
- unsigned long hva;
+ unsigned long hva, uninitialized_var(old_npages);
struct kvm_memslots *slots = kvm_memslots(kvm);
- struct kvm_memory_slot *slot, old;
+ struct kvm_memory_slot *slot;

/* Called with kvm->slots_lock held. */
if (WARN_ON(id >= KVM_MEM_SLOTS_NUM))
@@ -9542,7 +9542,7 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)

slot = id_to_memslot(slots, id);
if (size) {
- if (slot->npages)
+ if (slot && slot->npages)
return -EEXIST;

/*
@@ -9554,13 +9554,13 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
if (IS_ERR((void *)hva))
return PTR_ERR((void *)hva);
} else {
- if (!slot->npages)
+ if (!slot || !slot->npages)
return 0;

- hva = 0;
+ hva = slot->userspace_addr;
+ old_npages = slot->npages;
}

- old = *slot;
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
struct kvm_userspace_memory_region m;

@@ -9575,7 +9575,7 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
}

if (!size)
- vm_munmap(old.userspace_addr, old.npages * PAGE_SIZE);
+ vm_munmap(hva, old_npages * PAGE_SIZE);

return 0;
}
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 7d666eedd203..49b6b457a157 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -574,10 +574,11 @@ static inline int kvm_vcpu_get_idx(struct kvm_vcpu *vcpu)
return vcpu->vcpu_idx;
}

-#define kvm_for_each_memslot(memslot, slots) \
- for (memslot = &slots->memslots[0]; \
- memslot < slots->memslots + KVM_MEM_SLOTS_NUM && memslot->npages;\
- memslot++)
+#define kvm_for_each_memslot(memslot, slots) \
+ for (memslot = &slots->memslots[0]; \
+ memslot < slots->memslots + slots->used_slots; memslot++) \
+ if (WARN_ON_ONCE(!memslot->npages)) { \
+ } else

int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id);
void kvm_vcpu_uninit(struct kvm_vcpu *vcpu);
@@ -638,12 +639,15 @@ static inline struct kvm_memslots *kvm_vcpu_memslots(struct kvm_vcpu *vcpu)
return __kvm_memslots(vcpu->kvm, as_id);
}

-static inline struct kvm_memory_slot *
-id_to_memslot(struct kvm_memslots *slots, int id)
+static inline
+struct kvm_memory_slot *id_to_memslot(struct kvm_memslots *slots, int id)
{
int index = slots->id_to_index[id];
struct kvm_memory_slot *slot;

+ if (index < 0)
+ return NULL;
+
slot = &slots->memslots[index];

WARN_ON(slot->id != id);
@@ -1013,6 +1017,8 @@ bool kvm_arch_irqfd_allowed(struct kvm *kvm, struct kvm_irqfd *args);
* used in non-modular code in arch/powerpc/kvm/book3s_hv_rm_mmu.c.
* gfn_to_memslot() itself isn't here as an inline because that would
* bloat other code too much.
+ *
+ * IMPORTANT: Slots are sorted from highest GFN to lowest GFN!
*/
static inline struct kvm_memory_slot *
search_memslots(struct kvm_memslots *slots, gfn_t gfn)
diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index c49a3d5baad2..ae049c1033f3 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1536,8 +1536,13 @@ void kvm_mmu_wp_memory_region(struct kvm *kvm, int slot)
{
struct kvm_memslots *slots = kvm_memslots(kvm);
struct kvm_memory_slot *memslot = id_to_memslot(slots, slot);
- phys_addr_t start = memslot->base_gfn << PAGE_SHIFT;
- phys_addr_t end = (memslot->base_gfn + memslot->npages) << PAGE_SHIFT;
+ phys_addr_t start, end;
+
+ if (WARN_ON_ONCE(!memslot))
+ return;
+
+ start = memslot->base_gfn << PAGE_SHIFT;
+ end = (memslot->base_gfn + memslot->npages) << PAGE_SHIFT;

spin_lock(&kvm->mmu_lock);
stage2_wp_range(kvm, start, end);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 999a2a0c83f5..a1566c5cee26 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -569,7 +569,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
return NULL;

for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
- slots->id_to_index[i] = slots->memslots[i].id = i;
+ slots->id_to_index[i] = slots->memslots[i].id = -1;

return slots;
}
@@ -873,63 +873,162 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
}

/*
- * Insert memslot and re-sort memslots based on their GFN,
- * so binary search could be used to lookup GFN.
- * Sorting algorithm takes advantage of having initially
- * sorted array and known changed memslot position.
+ * Delete a memslot by decrementing the number of used slots and shifting all
+ * other entries in the array forward one spot.
+ */
+static inline void kvm_memslot_delete(struct kvm_memslots *slots,
+ struct kvm_memory_slot *memslot)
+{
+ struct kvm_memory_slot *mslots = slots->memslots;
+ int i;
+
+ if (WARN_ON(slots->id_to_index[memslot->id] == -1))
+ return;
+
+ slots->used_slots--;
+
+ for (i = slots->id_to_index[memslot->id]; i < slots->used_slots; i++) {
+ mslots[i] = mslots[i + 1];
+ slots->id_to_index[mslots[i].id] = i;
+ }
+ mslots[i] = *memslot;
+ slots->id_to_index[memslot->id] = -1;
+}
+
+/*
+ * "Insert" a new memslot by incrementing the number of used slots. Returns
+ * the new slot's initial index into the memslots array.
+ */
+static inline int kvm_memslot_insert_back(struct kvm_memslots *slots)
+{
+ return slots->used_slots++;
+}
+
+/*
+ * Move a changed memslot backwards in the array by shifting existing slots
+ * with a higher GFN toward the front of the array. Note, the changed memslot
+ * itself is not preserved in the array, i.e. not swapped at this time, only
+ * its new index into the array is tracked. Returns the changed memslot's
+ * current index into the memslots array.
+ */
+static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
+ struct kvm_memory_slot *memslot)
+{
+ struct kvm_memory_slot *mslots = slots->memslots;
+ int i;
+
+ if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
+ WARN_ON_ONCE(!slots->used_slots))
+ return -1;
+
+ /*
+ * Move the target memslot backward in the array by shifting existing
+ * memslots with a higher GFN (than the target memslot) towards the
+ * front of the array.
+ */
+ for (i = slots->id_to_index[memslot->id]; i < slots->used_slots - 1; i++) {
+ if (memslot->base_gfn > mslots[i + 1].base_gfn)
+ break;
+
+ WARN_ON_ONCE(memslot->base_gfn == mslots[i + 1].base_gfn);
+
+ /* Shift the next memslot forward one and update its index. */
+ mslots[i] = mslots[i + 1];
+ slots->id_to_index[mslots[i].id] = i;
+ }
+ return i;
+}
+
+/*
+ * Move a changed memslot forwards in the array by shifting existing slots with
+ * a lower GFN toward the back of the array. Note, the changed memslot itself
+ * is not preserved in the array, i.e. not swapped at this time, only its new
+ * index into the array is tracked. Returns the changed memslot's final index
+ * into the memslots array.
+ */
+static inline int kvm_memslot_move_forward(struct kvm_memslots *slots,
+ struct kvm_memory_slot *memslot,
+ int start)
+{
+ struct kvm_memory_slot *mslots = slots->memslots;
+ int i;
+
+ for (i = start; i > 0; i--) {
+ if (memslot->base_gfn < mslots[i - 1].base_gfn)
+ break;
+
+ WARN_ON_ONCE(memslot->base_gfn == mslots[i - 1].base_gfn);
+
+ /* Shift the next memslot back one and update its index. */
+ mslots[i] = mslots[i - 1];
+ slots->id_to_index[mslots[i].id] = i;
+ }
+ return i;
+}
+
+/*
+ * Re-sort memslots based on their GFN to account for an added, deleted, or
+ * moved memslot. Sorting memslots by GFN allows using a binary search during
+ * memslot lookup.
+ *
+ * IMPORTANT: Slots are sorted from highest GFN to lowest GFN! I.e. the entry
+ * at memslots[0] has the highest GFN.
+ *
+ * The sorting algorithm takes advantage of having initially sorted memslots
+ * and knowing the position of the changed memslot. Sorting is also optimized
+ * by not swapping the updated memslot and instead only shifting other memslots
+ * and tracking the new index for the update memslot. Only once its final
+ * index is known is the updated memslot copied into its position in the array.
+ *
+ * - When deleting a memslot, the deleted memslot simply needs to be moved to
+ * the end of the array.
+ *
+ * - When creating a memslot, the algorithm "inserts" the new memslot at the
+ * end of the array and then it forward to its correct location.
+ *
+ * - When moving a memslot, the algorithm first moves the updated memslot
+ * backward to handle the scenario where the memslot's GFN was changed to a
+ * lower value. update_memslots() then falls through and runs the same flow
+ * as creating a memslot to move the memslot forward to handle the scenario
+ * where its GFN was changed to a higher value.
+ *
+ * Note, slots are sorted from highest->lowest instead of lowest->highest for
+ * historical reasons. Originally, invalid memslots where denoted by having
+ * GFN=0, thus sorting from highest->lowest naturally sorted invalid memslots
+ * to the end of the array. The current algorithm uses dedicated logic to
+ * delete a memslot and thus does not rely on invalid memslots having GFN=0.
+ *
+ * The other historical motiviation for highest->lowest was to improve the
+ * performance of memslot lookup. KVM originally used a linear search starting
+ * at memslots[0]. On x86, the largest memslot usually has one of the highest,
+ * if not *the* highest, GFN, as the bulk of the guest's RAM is located in a
+ * single memslot above the 4gb boundary. As the largest memslot is also the
+ * most likely to be referenced, sorting it to the front of the array was
+ * advantageous. The current binary search starts from the middle of the array
+ * and uses an LRU pointer to improve performance for all memslots and GFNs.
*/
static void update_memslots(struct kvm_memslots *slots,
- struct kvm_memory_slot *new,
+ struct kvm_memory_slot *memslot,
enum kvm_mr_change change)
{
- int id = new->id;
- int i = slots->id_to_index[id];
- struct kvm_memory_slot *mslots = slots->memslots;
+ int i;

- WARN_ON(mslots[i].id != id);
- switch (change) {
- case KVM_MR_CREATE:
- slots->used_slots++;
- WARN_ON(mslots[i].npages || !new->npages);
- break;
- case KVM_MR_DELETE:
- slots->used_slots--;
- WARN_ON(new->npages || !mslots[i].npages);
- break;
- default:
- break;
- }
+ if (change == KVM_MR_DELETE) {
+ kvm_memslot_delete(slots, memslot);
+ } else {
+ if (change == KVM_MR_CREATE)
+ i = kvm_memslot_insert_back(slots);
+ else
+ i = kvm_memslot_move_backward(slots, memslot);
+ i = kvm_memslot_move_forward(slots, memslot, i);

- while (i < KVM_MEM_SLOTS_NUM - 1 &&
- new->base_gfn <= mslots[i + 1].base_gfn) {
- if (!mslots[i + 1].npages)
- break;
- mslots[i] = mslots[i + 1];
- slots->id_to_index[mslots[i].id] = i;
- i++;
+ /*
+ * Copy the memslot to its new position in memslots and update
+ * its index accordingly.
+ */
+ slots->memslots[i] = *memslot;
+ slots->id_to_index[memslot->id] = i;
}
-
- /*
- * The ">=" is needed when creating a slot with base_gfn == 0,
- * so that it moves before all those with base_gfn == npages == 0.
- *
- * On the other hand, if new->npages is zero, the above loop has
- * already left i pointing to the beginning of the empty part of
- * mslots, and the ">=" would move the hole backwards in this
- * case---which is wrong. So skip the loop when deleting a slot.
- */
- if (new->npages) {
- while (i > 0 &&
- new->base_gfn >= mslots[i - 1].base_gfn) {
- mslots[i] = mslots[i - 1];
- slots->id_to_index[mslots[i].id] = i;
- i--;
- }
- } else
- WARN_ON_ONCE(i != slots->used_slots);
-
- mslots[i] = *new;
- slots->id_to_index[mslots[i].id] = i;
}

static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
@@ -1108,8 +1207,13 @@ int __kvm_set_memory_region(struct kvm *kvm,
* when the memslots are re-sorted by update_memslots().
*/
tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
- old = *tmp;
- tmp = NULL;
+ if (tmp) {
+ old = *tmp;
+ tmp = NULL;
+ } else {
+ memset(&old, 0, sizeof(old));
+ old.id = id;
+ }

if (!mem->memory_size)
return kvm_delete_memslot(kvm, mem, &old, as_id);
@@ -1227,7 +1331,7 @@ int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,

slots = __kvm_memslots(kvm, as_id);
*memslot = id_to_memslot(slots, id);
- if (!(*memslot)->dirty_bitmap)
+ if (!(*memslot) || !(*memslot)->dirty_bitmap)
return -ENOENT;

kvm_arch_sync_dirty_log(kvm, *memslot);
@@ -1285,10 +1389,10 @@ static int kvm_get_dirty_log_protect(struct kvm *kvm, struct kvm_dirty_log *log)

slots = __kvm_memslots(kvm, as_id);
memslot = id_to_memslot(slots, id);
+ if (!memslot || !memslot->dirty_bitmap)
+ return -ENOENT;

dirty_bitmap = memslot->dirty_bitmap;
- if (!dirty_bitmap)
- return -ENOENT;

kvm_arch_sync_dirty_log(kvm, memslot);

@@ -1396,10 +1500,10 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,

slots = __kvm_memslots(kvm, as_id);
memslot = id_to_memslot(slots, id);
+ if (!memslot || !memslot->dirty_bitmap)
+ return -ENOENT;

dirty_bitmap = memslot->dirty_bitmap;
- if (!dirty_bitmap)
- return -ENOENT;

n = ALIGN(log->num_pages, BITS_PER_LONG) / 8;

--
2.24.1
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