KVM: doc: Document the life cycle of a VM and its resources

The series to add memcg accounting to KVM allocations[1] states:

  There are many KVM kernel memory allocations which are tied to the
  life of the VM process and should be charged to the VM process's
  cgroup.

While it is correct to account KVM kernel allocations to the cgroup of
the process that created the VM, it's technically incorrect to state
that the KVM kernel memory allocations are tied to the life of the VM
process.  This is because the VM itself, i.e. struct kvm, is not tied to
the life of the process which created it, rather it is tied to the life
of its associated file descriptor.  In other words, kvm_destroy_vm() is
not invoked until fput() decrements its associated file's refcount to
zero.  A simple example is to fork() in Qemu and have the child sleep
indefinitely; kvm_destroy_vm() isn't called until Qemu closes its file
descriptor *and* the rogue child is killed.

The allocations are guaranteed to be *accounted* to the process which
created the VM, but only because KVM's per-{VM,vCPU} ioctls reject the
ioctl() with -EIO if kvm->mm != current->mm.  I.e. the child can keep
the VM "alive" but can't do anything useful with its reference.

Note that because 'struct kvm' also holds a reference to the mm_struct
of its owner, the above behavior also applies to userspace allocations.

Given that mucking with a VM's file descriptor can lead to subtle and
undesirable behavior, e.g. memcg charges persisting after a VM is shut
down, explicitly document a VM's lifecycle and its impact on the VM's
resources.

Alternatively, KVM could aggressively free resources when the creating
process exits, e.g. via mmu_notifier->release().  However, mmu_notifier
isn't guaranteed to be available, and freeing resources when the creator
exits is likely to be error prone and fragile as KVM would need to
ensure that it only freed resources that are truly out of reach. In
practice, the existing behavior shouldn't be problematic as a properly
configured system will prevent a child process from being moved out of
the appropriate cgroup hierarchy, i.e. prevent hiding the process from
the OOM killer, and will prevent an unprivileged user from being able to
to hold a reference to struct kvm via another method, e.g. debugfs.

[1]https://patchwork.kernel.org/patch/10806707/

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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