Documentation: kvm: clarify SRCU locking order

Currently only the locking order of SRCU vs kvm->slots_arch_lock
and kvm->slots_lock is documented.  Extend this to kvm->lock
since Xen emulation got it terribly wrong.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-12-28 06:00:22 -05:00
parent a79b53aaaa
commit 02d9a04da4

View File

@ -16,17 +16,26 @@ The acquisition orders for mutexes are as follows:
- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
them together is quite rare.
- Unlike kvm->slots_lock, kvm->slots_arch_lock is released before
synchronize_srcu(&kvm->srcu). Therefore kvm->slots_arch_lock
can be taken inside a kvm->srcu read-side critical section,
while kvm->slots_lock cannot.
- kvm->mn_active_invalidate_count ensures that pairs of
invalidate_range_start() and invalidate_range_end() callbacks
use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock
are taken on the waiting side in install_new_memslots, so MMU notifiers
must not take either kvm->slots_lock or kvm->slots_arch_lock.
For SRCU:
- ``synchronize_srcu(&kvm->srcu)`` is called _inside_
the kvm->slots_lock critical section, therefore kvm->slots_lock
cannot be taken inside a kvm->srcu read-side critical section.
Instead, kvm->slots_arch_lock is released before the call
to ``synchronize_srcu()`` and _can_ be taken inside a
kvm->srcu read-side critical section.
- kvm->lock is taken inside kvm->srcu, therefore
``synchronize_srcu(&kvm->srcu)`` cannot be called inside
a kvm->lock critical section. If you cannot delay the
call until after kvm->lock is released, use ``call_srcu``.
On x86:
- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock