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Message-ID: <20240926013506.860253-10-jthoughton@google.com>
Date: Thu, 26 Sep 2024 01:34:57 +0000
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Matlack <dmatlack@...gle.com>,
David Rientjes <rientjes@...gle.com>, James Houghton <jthoughton@...gle.com>,
Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>,
Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v7 09/18] KVM: x86/mmu: Add support for lockless walks of rmap SPTEs
From: Sean Christopherson <seanjc@...gle.com>
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: Sean Christopherson <seanjc@...gle.com>
[jthoughton: Added lockdep assertion for kvm_rmap_lock, synchronization fixup]
Signed-off-by: James Houghton <jthoughton@...gle.com>
---
arch/x86/kvm/mmu/mmu.c | 75 +++++++++++++++++++++++-------------------
1 file changed, 42 insertions(+), 33 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 79676798ba77..72c682fa207a 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -932,7 +932,7 @@ 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;
@@ -976,14 +976,25 @@ 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 unsigned long kvm_rmap_lock(struct kvm *kvm,
+ struct kvm_rmap_head *rmap_head)
+{
+ 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 new_val)
{
- WARN_ON_ONCE(new_val & KVM_RMAP_LOCKED);
+ KVM_MMU_WARN_ON(new_val & KVM_RMAP_LOCKED);
/*
* Ensure that all accesses to the rmap have completed
* before we actually unlock the rmap.
@@ -1023,14 +1034,14 @@ static void kvm_rmap_unlock_readonly(struct kvm_rmap_head *rmap_head,
/*
* 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;
@@ -1110,7 +1121,7 @@ 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;
@@ -1154,7 +1165,7 @@ 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;
@@ -1246,23 +1257,18 @@ 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];
}
/*
@@ -1272,14 +1278,11 @@ 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;
@@ -1287,20 +1290,24 @@ 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)
{
@@ -1396,11 +1403,12 @@ 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 |= spte_wrprot_for_clear_dirty(sptep);
else
flush |= spte_clear_dirty(sptep);
+ }
return flush;
}
@@ -1710,7 +1718,7 @@ 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;
@@ -1859,13 +1867,14 @@ 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,
@@ -2555,7 +2564,7 @@ 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
--
2.46.0.792.g87dc391469-goog
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