On 02/04/21 02:56, Sean Christopherson wrote:
Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}()
notifications. Because mmu_notifier_count must be modified while holding
mmu_lock for write, and must always be paired across start->end to stay
balanced, lock elision must happen in both or none. To meet that
requirement, add a rwsem to prevent memslot updates across range_start()
and range_end().
Use a rwsem instead of a rwlock since most notifiers _allow_ blocking,
and the lock will be endl across the entire start() ... end() sequence.
If anything in the sequence sleeps, including the caller or a different
notifier, holding the spinlock would be disastrous.
For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down
the slow path of unconditionally acquiring mmu_lock. The sane
alternative would be to try to acquire the lock and force the notifier
to retry on failure. But since OOM is currently the _only_ scenario
where blocking is disallowed attempting to optimize a guest that has been
marked for death is pointless.
Unconditionally define and use mmu_notifier_slots_lock in the memslots
code, purely to avoid more #ifdefs. The overhead of acquiring the lock
is negligible when the lock is uncontested, which will always be the case
when the MMU notifiers are not used.
Note, technically flag-only memslot updates could be allowed in parallel,
but stalling a memslot update for a relatively short amount of time is
not a scalability issue, and this is all more than complex enough.
Proposal for the locking documentation:
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index b21a34c34a21..3e4ad7de36cb 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -16,6 +16,13 @@ The acquisition orders for mutexes are as follows:
- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
them together is quite rare.
+- The kvm->mmu_notifier_slots_lock rwsem ensures that pairs of
+ invalidate_range_start() and invalidate_range_end() callbacks
+ use the same memslots array. kvm->slots_lock is taken outside the
+ write-side critical section of kvm->mmu_notifier_slots_lock, so
+ MMU notifiers must not take kvm->slots_lock. No other write-side
+ critical sections should be added.
+
On x86, vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock.
Everything else is a leaf: no other lock is taken inside the critical
Paolo
