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

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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: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250204004038.1680123-11-jthoughton@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2025-02-04 00:40:37 +00:00
parent 4834eaded9
commit bb6c7749cc
1 changed files with 92 additions and 41 deletions
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133
arch/x86/kvm/mmu/mmu.c
View File

@ -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