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KVM: x86/mmu: Allow yielding when zapping GFNs for defunct TDP MMU root 

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().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-03-03 01:50:21 -05:00
parent 22b94c4b63
commit 8351779ce6
1 changed files with 62 additions and 41 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

103
arch/x86/kvm/mmu/tdp_mmu.c
View File

@ -124,6 +124,16 @@ static void tdp_mmu_schedule_zap_root(struct kvm *kvm, struct kvm_mmu_page *root
queue_work(kvm->arch.tdp_mmu_zap_wq, &root->tdp_mmu_async_work);
}
static inline bool kvm_tdp_root_mark_invalid(struct kvm_mmu_page *page)
{
union kvm_mmu_page_role role = page->role;
role.invalid = true;
/* No need to use cmpxchg, only the invalid bit can change. */
role.word = xchg(&page->role.word, role.word);
return role.invalid;
}
void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
bool shared)
{
@ -134,20 +144,46 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
WARN_ON(!root->tdp_mmu_page);
/*
* 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 if 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.
*/
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);
/*
* 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.
*/
tdp_mmu_zap_root(kvm, root, shared);
call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
}
@ -789,12 +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();
@ -805,42 +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();