Documentation: KVM: Formalizing taking vcpu->mutex *outside* of kvm->slots_lock

Explicitly document the ordering of vcpu->mutex being taken *outside* of
kvm->slots_lock.  While somewhat unintuitive since vCPUs conceptually have
narrower scope than VMs, the scope of the owning object (vCPU versus VM)
doesn't automatically carry over to the lock.  In this case, vcpu->mutex
has far broader scope than kvm->slots_lock.  As Paolo put it, it's a
"don't worry about multiple ioctls at the same time" mutex that's intended
to be taken at the outer edges of KVM.

More importantly, arm64 and x86 have gained flows that take kvm->slots_lock
inside of vcpu->mutex.  x86's kvm_inhibit_apic_access_page() is particularly
nasty, as slots_lock is taken quite deep within KVM_RUN, i.e. simply
swapping the ordering isn't an option.

Commit to the vcpu->mutex => kvm->slots_lock ordering, as vcpu->mutex
really is intended to be a "top-level" lock, whereas kvm->slots_lock is
"just" a helper lock.

Opportunistically document that vcpu->mutex is also taken outside of
slots_arch_lock, e.g. when allocating shadow roots on x86 (which is the
entire reason slots_arch_lock exists, as shadow roots must be allocated
while holding kvm->srcu)

  kvm_mmu_new_pgd()
  |
  -> kvm_mmu_reload()
     |
     -> kvm_mmu_load()
        |
        -> mmu_alloc_shadow_roots()
           |
           -> mmu_first_shadow_root_alloc()

but also when manipulating memslots in vCPU context, e.g. when inhibiting
the APIC-access page via the aforementioned kvm_inhibit_apic_access_page()

  kvm_inhibit_apic_access_page()
  |
  -> __x86_set_memory_region()
     |
     -> kvm_set_internal_memslot()
        |
        -> kvm_set_memory_region()
           |
           -> kvm_set_memslot()

Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://patch.msgid.link/20260302170239.596810-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

Documentation/virt/kvm/locking.rst

@@ -17,6 +17,8 @@ The acquisition orders for mutexes are as follows:
 
 - kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock
 
+- vcpu->mutex is taken outside kvm->slots_lock and kvm->slots_arch_lock
+
 - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
   them together is quite rare.