Commit bb6c7749 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

KVM: x86/mmu: Add support for lockless walks of rmap SPTEs 



Add a lockless version of for_each_rmap_spte(), which is pretty much the
same as the normal version, except that it doesn't BUG() the host if a
non-present SPTE is encountered.  When mmu_lock is held, it should be
impossible for a different task to zap a SPTE, _and_ zapped SPTEs must
be removed from their rmap chain prior to dropping mmu_lock.  Thus, the
normal walker BUG()s if a non-present SPTE is encountered as something is
wildly broken.

When walking rmaps without holding mmu_lock, the SPTEs pointed at by the
rmap chain can be zapped/dropped, and so a lockless walk can observe a
non-present SPTE if it runs concurrently with a different operation that
is zapping SPTEs.

Signed-off-by: default avatarJames Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250204004038.1680123-11-jthoughton@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 4834eade

arch/x86/kvm/mmu/mmu.c

+92 −41
Original line number Diff line number Diff line
@@ -876,7 +876,7 @@ static struct kvm_memory_slot *gfn_to_memslot_dirty_bitmap(struct kvm_vcpu *vcpu 
 */

#define KVM_RMAP_LOCKED	BIT(1)



static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head)

static unsigned long __kvm_rmap_lock(struct kvm_rmap_head *rmap_head)

{

	unsigned long old_val, new_val;
@@ -914,7 +914,7 @@ static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head) 
	/*

	 * Use try_cmpxchg_acquire() to prevent reads and writes to the rmap

	 * from being reordered outside of the critical section created by

	 * kvm_rmap_lock().

	 * __kvm_rmap_lock().

	 *

	 * Pairs with the atomic_long_set_release() in kvm_rmap_unlock().

	 *
@@ -923,21 +923,42 @@ static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head) 
	 */

	} while (!atomic_long_try_cmpxchg_acquire(&rmap_head->val, &old_val, new_val));



	/* Return the old value, i.e. _without_ the LOCKED bit set. */

	/*

	 * Return the old value, i.e. _without_ the LOCKED bit set.  It's

	 * impossible for the return value to be 0 (see above), i.e. the read-

	 * only unlock flow can't get a false positive and fail to unlock.

	 */

	return old_val;

}



static void kvm_rmap_unlock(struct kvm_rmap_head *rmap_head,

			    unsigned long new_val)

static unsigned long kvm_rmap_lock(struct kvm *kvm,

				   struct kvm_rmap_head *rmap_head)

{

	WARN_ON_ONCE(new_val & KVM_RMAP_LOCKED);

	lockdep_assert_held_write(&kvm->mmu_lock);



	return __kvm_rmap_lock(rmap_head);

}



static void __kvm_rmap_unlock(struct kvm_rmap_head *rmap_head,

			      unsigned long val)

{

	KVM_MMU_WARN_ON(val & KVM_RMAP_LOCKED);

	/*

	 * Ensure that all accesses to the rmap have completed before unlocking

	 * the rmap.

	 *

	 * Pairs with the atomic_long_try_cmpxchg_acquire() in kvm_rmap_lock.

	 * Pairs with the atomic_long_try_cmpxchg_acquire() in __kvm_rmap_lock().

	 */

	atomic_long_set_release(&rmap_head->val, new_val);

	atomic_long_set_release(&rmap_head->val, val);

}



static void kvm_rmap_unlock(struct kvm *kvm,

			    struct kvm_rmap_head *rmap_head,

			    unsigned long new_val)

{

	lockdep_assert_held_write(&kvm->mmu_lock);



	__kvm_rmap_unlock(rmap_head, new_val);

}



static unsigned long kvm_rmap_get(struct kvm_rmap_head *rmap_head)
@@ -945,17 +966,49 @@ static unsigned long kvm_rmap_get(struct kvm_rmap_head *rmap_head) 
	return atomic_long_read(&rmap_head->val) & ~KVM_RMAP_LOCKED;

}



/*

 * If mmu_lock isn't held, rmaps can only be locked in read-only mode.  The

 * actual locking is the same, but the caller is disallowed from modifying the

 * rmap, and so the unlock flow is a nop if the rmap is/was empty.

 */

__maybe_unused

static unsigned long kvm_rmap_lock_readonly(struct kvm_rmap_head *rmap_head)

{

	unsigned long rmap_val;



	preempt_disable();

	rmap_val = __kvm_rmap_lock(rmap_head);



	if (!rmap_val)

		preempt_enable();



	return rmap_val;

}



__maybe_unused

static void kvm_rmap_unlock_readonly(struct kvm_rmap_head *rmap_head,

				     unsigned long old_val)

{

	if (!old_val)

		return;



	KVM_MMU_WARN_ON(old_val != kvm_rmap_get(rmap_head));



	__kvm_rmap_unlock(rmap_head, old_val);

	preempt_enable();

}



/*

 * Returns the number of pointers in the rmap chain, not counting the new one.

 */

static int pte_list_add(struct kvm_mmu_memory_cache *cache, u64 *spte,

			struct kvm_rmap_head *rmap_head)

static int pte_list_add(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,

			u64 *spte, struct kvm_rmap_head *rmap_head)

{

	unsigned long old_val, new_val;

	struct pte_list_desc *desc;

	int count = 0;



	old_val = kvm_rmap_lock(rmap_head);

	old_val = kvm_rmap_lock(kvm, rmap_head);



	if (!old_val) {

		new_val = (unsigned long)spte;
@@ -987,7 +1040,7 @@ static int pte_list_add(struct kvm_mmu_memory_cache *cache, u64 *spte, 
		desc->sptes[desc->spte_count++] = spte;

	}



	kvm_rmap_unlock(rmap_head, new_val);

	kvm_rmap_unlock(kvm, rmap_head, new_val);



	return count;

}
@@ -1035,7 +1088,7 @@ static void pte_list_remove(struct kvm *kvm, u64 *spte, 
	unsigned long rmap_val;

	int i;



	rmap_val = kvm_rmap_lock(rmap_head);

	rmap_val = kvm_rmap_lock(kvm, rmap_head);

	if (KVM_BUG_ON_DATA_CORRUPTION(!rmap_val, kvm))

		goto out;
@@ -1061,7 +1114,7 @@ static void pte_list_remove(struct kvm *kvm, u64 *spte, 
	}



out:

	kvm_rmap_unlock(rmap_head, rmap_val);

	kvm_rmap_unlock(kvm, rmap_head, rmap_val);

}



static void kvm_zap_one_rmap_spte(struct kvm *kvm,
@@ -1079,7 +1132,7 @@ static bool kvm_zap_all_rmap_sptes(struct kvm *kvm, 
	unsigned long rmap_val;

	int i;



	rmap_val = kvm_rmap_lock(rmap_head);

	rmap_val = kvm_rmap_lock(kvm, rmap_head);

	if (!rmap_val)

		return false;
@@ -1098,7 +1151,7 @@ static bool kvm_zap_all_rmap_sptes(struct kvm *kvm, 
	}

out:

	/* rmap_head is meaningless now, remember to reset it */

	kvm_rmap_unlock(rmap_head, 0);

	kvm_rmap_unlock(kvm, rmap_head, 0);

	return true;

}
@@ -1171,23 +1224,18 @@ static u64 *rmap_get_first(struct kvm_rmap_head *rmap_head, 
			   struct rmap_iterator *iter)

{

	unsigned long rmap_val = kvm_rmap_get(rmap_head);

	u64 *sptep;



	if (!rmap_val)

		return NULL;



	if (!(rmap_val & KVM_RMAP_MANY)) {

		iter->desc = NULL;

		sptep = (u64 *)rmap_val;

		goto out;

		return (u64 *)rmap_val;

	}



	iter->desc = (struct pte_list_desc *)(rmap_val & ~KVM_RMAP_MANY);

	iter->pos = 0;

	sptep = iter->desc->sptes[iter->pos];

out:

	BUG_ON(!is_shadow_present_pte(*sptep));

	return sptep;

	return iter->desc->sptes[iter->pos];

}



/*
@@ -1197,14 +1245,11 @@ static u64 *rmap_get_first(struct kvm_rmap_head *rmap_head, 
 */

static u64 *rmap_get_next(struct rmap_iterator *iter)

{

	u64 *sptep;



	if (iter->desc) {

		if (iter->pos < PTE_LIST_EXT - 1) {

			++iter->pos;

			sptep = iter->desc->sptes[iter->pos];

			if (sptep)

				goto out;

			if (iter->desc->sptes[iter->pos])

				return iter->desc->sptes[iter->pos];

		}



		iter->desc = iter->desc->more;
@@ -1212,20 +1257,24 @@ static u64 *rmap_get_next(struct rmap_iterator *iter) 
		if (iter->desc) {

			iter->pos = 0;

			/* desc->sptes[0] cannot be NULL */

			sptep = iter->desc->sptes[iter->pos];

			goto out;

			return iter->desc->sptes[iter->pos];

		}

	}



	return NULL;

out:

	BUG_ON(!is_shadow_present_pte(*sptep));

	return sptep;

}



#define for_each_rmap_spte(_rmap_head_, _iter_, _spte_)			\

	for (_spte_ = rmap_get_first(_rmap_head_, _iter_);		\

	     _spte_; _spte_ = rmap_get_next(_iter_))

#define __for_each_rmap_spte(_rmap_head_, _iter_, _sptep_)	\

	for (_sptep_ = rmap_get_first(_rmap_head_, _iter_);	\

	     _sptep_; _sptep_ = rmap_get_next(_iter_))



#define for_each_rmap_spte(_rmap_head_, _iter_, _sptep_)			\

	__for_each_rmap_spte(_rmap_head_, _iter_, _sptep_)			\

		if (!WARN_ON_ONCE(!is_shadow_present_pte(*(_sptep_))))	\



#define for_each_rmap_spte_lockless(_rmap_head_, _iter_, _sptep_, _spte_)	\

	__for_each_rmap_spte(_rmap_head_, _iter_, _sptep_)			\

		if (is_shadow_present_pte(_spte_ = mmu_spte_get_lockless(sptep)))



static void drop_spte(struct kvm *kvm, u64 *sptep)

{
@@ -1311,12 +1360,13 @@ static bool __rmap_clear_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head, 
	struct rmap_iterator iter;

	bool flush = false;



	for_each_rmap_spte(rmap_head, &iter, sptep)

	for_each_rmap_spte(rmap_head, &iter, sptep) {

		if (spte_ad_need_write_protect(*sptep))

			flush |= test_and_clear_bit(PT_WRITABLE_SHIFT,

						    (unsigned long *)sptep);

		else

			flush |= spte_clear_dirty(sptep);

	}



	return flush;

}
@@ -1637,7 +1687,7 @@ static void __rmap_add(struct kvm *kvm, 
	kvm_update_page_stats(kvm, sp->role.level, 1);



	rmap_head = gfn_to_rmap(gfn, sp->role.level, slot);

	rmap_count = pte_list_add(cache, spte, rmap_head);

	rmap_count = pte_list_add(kvm, cache, spte, rmap_head);



	if (rmap_count > kvm->stat.max_mmu_rmap_size)

		kvm->stat.max_mmu_rmap_size = rmap_count;
@@ -1771,13 +1821,14 @@ static unsigned kvm_page_table_hashfn(gfn_t gfn) 
	return hash_64(gfn, KVM_MMU_HASH_SHIFT);

}



static void mmu_page_add_parent_pte(struct kvm_mmu_memory_cache *cache,

static void mmu_page_add_parent_pte(struct kvm *kvm,

				    struct kvm_mmu_memory_cache *cache,

				    struct kvm_mmu_page *sp, u64 *parent_pte)

{

	if (!parent_pte)

		return;



	pte_list_add(cache, parent_pte, &sp->parent_ptes);

	pte_list_add(kvm, cache, parent_pte, &sp->parent_ptes);

}



static void mmu_page_remove_parent_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
@@ -2467,7 +2518,7 @@ static void __link_shadow_page(struct kvm *kvm, 


	mmu_spte_set(sptep, spte);



	mmu_page_add_parent_pte(cache, sp, sptep);

	mmu_page_add_parent_pte(kvm, cache, sp, sptep);



	/*

	 * The non-direct sub-pagetable must be updated before linking.  For