Allow yielding when zapping SPTEs after the last reference to a valid
root is put. Because KVM must drop all SPTEs in response to relevant
mmu_notifier events, mark defunct roots invalid and reset their refcount
prior to zapping the root. Keeping the refcount elevated while the zap
is in-progress ensures the root is reachable via mmu_notifier until the
zap completes and the last reference to the invalid, defunct root is put.
Allowing kvm_tdp_mmu_put_root() to yield fixes soft lockup issues if the
root in being put has a massive paging structure, e.g. zapping a root
that is backed entirely by 4kb pages for a guest with 32tb of memory can
take hundreds of seconds to complete.
watchdog: BUG: soft lockup - CPU#49 stuck for 485s! [max_guest_memor:52368]
RIP: 0010:kvm_set_pfn_dirty+0x30/0x50 [kvm]
__handle_changed_spte+0x1b2/0x2f0 [kvm]
handle_removed_tdp_mmu_page+0x1a7/0x2b8 [kvm]
__handle_changed_spte+0x1f4/0x2f0 [kvm]
handle_removed_tdp_mmu_page+0x1a7/0x2b8 [kvm]
__handle_changed_spte+0x1f4/0x2f0 [kvm]
tdp_mmu_zap_root+0x307/0x4d0 [kvm]
kvm_tdp_mmu_put_root+0x7c/0xc0 [kvm]
kvm_mmu_free_roots+0x22d/0x350 [kvm]
kvm_mmu_reset_context+0x20/0x60 [kvm]
kvm_arch_vcpu_ioctl_set_sregs+0x5a/0xc0 [kvm]
kvm_vcpu_ioctl+0x5bd/0x710 [kvm]
__se_sys_ioctl+0x77/0xc0
__x64_sys_ioctl+0x1d/0x20
do_syscall_64+0x44/0xa0
entry_SYSCALL_64_after_hwframe+0x44/0xae
KVM currently doesn't put a root from a non-preemptible context, so other
than the mmu_notifier wrinkle, yielding when putting a root is safe.
Yield-unfriendly iteration uses for_each_tdp_mmu_root(), which doesn't
take a reference to each root (it requires mmu_lock be held for the
entire duration of the walk).
tdp_mmu_next_root() is used only by the yield-friendly iterator.
tdp_mmu_zap_root_work() is explicitly yield friendly.
kvm_mmu_free_roots() => mmu_free_root_page() is a much bigger fan-out,
but is still yield-friendly in all call sites, as all callers can be
traced back to some combination of vcpu_run(), kvm_destroy_vm(), and/or
kvm_create_vm().
Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-Id: <20220226001546.360188-21-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/mmu/tdp_mmu.c | 93 +++++++++++++++++++++-----------------
1 file changed, 52 insertions(+), 41 deletions(-)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index ed1bb63b342d..408e21e4009c 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -144,20 +144,46 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
WARN_ON(!root->tdp_mmu_page);
- spin_lock(&kvm->arch.tdp_mmu_pages_lock);
- list_del_rcu(&root->link);
- spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
-
/*
- * A TLB flush is not necessary as KVM performs a local TLB flush when
- * allocating a new root (see kvm_mmu_load()), and when migrating vCPU
- * to a different pCPU. Note, the local TLB flush on reuse also
- * invalidates any paging-structure-cache entries, i.e. TLB entries for
- * intermediate paging structures, that may be zapped, as such entries
- * are associated with the ASID on both VMX and SVM.
+ * The root now has refcount=0. It is valid, but readers already
+ * cannot acquire a reference to it because kvm_tdp_mmu_get_root()
+ * rejects it. This remains true for the rest of the execution
+ * of this function, because readers visit valid roots only
+ * (except for tdp_mmu_zap_root_work(), which however
+ * does not acquire any reference itself).
+ *
+ * Even though there are flows that need to visit all roots for
+ * correctness, they all take mmu_lock for write, so they cannot yet
+ * run concurrently. The same is true after kvm_tdp_root_mark_invalid,
+ * since the root still has refcount=0.
+ *
+ * However, tdp_mmu_zap_root can yield, and writers do not expect to
+ * see refcount=0 (see for example kvm_tdp_mmu_invalidate_all_roots()).
+ * So the root temporarily gets an extra reference, going to refcount=1
+ * while staying invalid. Readers still cannot acquire any reference;
+ * but writers are now allowed to run if tdp_mmu_zap_root yields and
+ * they might take an extra reference is they themselves yield. Therefore,
+ * when the reference is given back after tdp_mmu_zap_root terminates,
+ * there is no guarantee that the refcount is still 1. If not, whoever
+ * puts the last reference will free the page, but they will not have to
+ * zap the root because a root cannot go from invalid to valid.
*/
- tdp_mmu_zap_root(kvm, root, shared);
+ if (!kvm_tdp_root_mark_invalid(root)) {
+ refcount_set(&root->tdp_mmu_root_count, 1);
+ tdp_mmu_zap_root(kvm, root, shared);
+
+ /*
+ * Give back the reference that was added back above. We now
+ * know that the root is invalid, so go ahead and free it if
+ * no one has taken a reference in the meanwhile.
+ */
+ if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
+ return;
+ }
+ spin_lock(&kvm->arch.tdp_mmu_pages_lock);
+ list_del_rcu(&root->link);
+ spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
}
@@ -799,12 +825,23 @@ static inline gfn_t tdp_mmu_max_gfn_host(void)
static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
bool shared)
{
- bool root_is_unreachable = !refcount_read(&root->tdp_mmu_root_count);
struct tdp_iter iter;
gfn_t end = tdp_mmu_max_gfn_host();
gfn_t start = 0;
+ /*
+ * The root must have an elevated refcount so that it's reachable via
+ * mmu_notifier callbacks, which allows this path to yield and drop
+ * mmu_lock. When handling an unmap/release mmu_notifier command, KVM
+ * must drop all references to relevant pages prior to completing the
+ * callback. Dropping mmu_lock with an unreachable root would result
+ * in zapping SPTEs after a relevant mmu_notifier callback completes
+ * and lead to use-after-free as zapping a SPTE triggers "writeback" of
+ * dirty accessed bits to the SPTE's associated struct page.
+ */
+ WARN_ON_ONCE(!refcount_read(&root->tdp_mmu_root_count));
+
kvm_lockdep_assert_mmu_lock_held(kvm, shared);
rcu_read_lock();
@@ -815,42 +852,16 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
*/
for_each_tdp_pte_min_level(iter, root, root->role.level, start, end) {
retry:
- /*
- * Yielding isn't allowed when zapping an unreachable root as
- * the root won't be processed by mmu_notifier callbacks. When
- * handling an unmap/release mmu_notifier command, KVM must
- * drop all references to relevant pages prior to completing
- * the callback. Dropping mmu_lock can result in zapping SPTEs
- * for an unreachable root after a relevant callback completes,
- * which leads to use-after-free as zapping a SPTE triggers
- * "writeback" of dirty/accessed bits to the SPTE's associated
- * struct page.
- */
- if (!root_is_unreachable &&
- tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
+ if (tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
continue;
if (!is_shadow_present_pte(iter.old_spte))
continue;
- if (!shared) {
+ if (!shared)
tdp_mmu_set_spte(kvm, &iter, 0);
- } else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0)) {
- /*
- * cmpxchg() shouldn't fail if the root is unreachable.
- * Retry so as not to leak the page and its children.
- */
- WARN_ONCE(root_is_unreachable,
- "Contended TDP MMU SPTE in unreachable root.");
+ else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0))
goto retry;
- }
-
- /*
- * WARN if the root is invalid and is unreachable, all SPTEs
- * should've been zapped by kvm_tdp_mmu_zap_invalidated_roots(),
- * and inserting new SPTEs under an invalid root is a KVM bug.
- */
- WARN_ON_ONCE(root_is_unreachable && root->role.invalid);
}
rcu_read_unlock();
--
2.31.1
