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Message-ID: <20240809194335.1726916-21-seanjc@google.com>
Date: Fri, 9 Aug 2024 12:43:32 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>, Peter Xu <peterx@...hat.com>,
James Houghton <jthoughton@...gle.com>
Subject: [PATCH 20/22] 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: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/kvm/mmu/mmu.c | 86 +++++++++++++++++++++++++++++++-----------
1 file changed, 63 insertions(+), 23 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a683b5fc4026..48e8608c2738 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -932,10 +932,16 @@ 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;
+ /*
+ * Elide the lock if the rmap is empty, as lockless walkers (read-only
+ * mode) don't need to (and can't) walk an empty rmap, nor can they add
+ * entries to the rmap. I.e. the only paths that process empty rmaps
+ * do so while holding mmu_lock for write, and are mutually exclusive.
+ */
old_val = READ_ONCE(rmap_head->val);
if (!old_val)
return 0;
@@ -960,17 +966,53 @@ static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head)
new_val = old_val | KVM_RMAP_LOCKED;
} while (!try_cmpxchg(&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_rmap_head *rmap_head)
+{
+ /*
+ * TODO: Plumb in @kvm and add a lockdep assertion that mmu_lock is
+ * held for write.
+ */
+ 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);
WRITE_ONCE(rmap_head->val, new_val);
}
+/*
+ * If mmu_lock isn't held, rmaps can only 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)
+{
+ return __kvm_rmap_lock(rmap_head);
+}
+
+__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 != (rmap_head->val & ~KVM_RMAP_LOCKED));
+ WRITE_ONCE(rmap_head->val, old_val);
+}
+
+
static unsigned long kvm_rmap_get(struct kvm_rmap_head *rmap_head)
{
return READ_ONCE(rmap_head->val) & ~KVM_RMAP_LOCKED;
@@ -1202,23 +1244,18 @@ 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];
}
/*
@@ -1228,14 +1265,11 @@ 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;
@@ -1243,20 +1277,26 @@ 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 (!is_shadow_present_pte(*(_sptep_))) \
+ BUG(); \
+ else
+
+#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)
{
--
2.46.0.76.ge559c4bf1a-goog
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