linux-kernel - Re: [PATCH 04/13] KVM: WARN if there are danging MMU invalidations at VM destruction

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Date:   Thu, 28 Sep 2023 11:11:24 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Binbin Wu <binbin.wu@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Michael Roth <michael.roth@....com>
Subject: Re: [PATCH 04/13] KVM: WARN if there are danging MMU invalidations at
 VM destruction

On Wed, Sep 27, 2023, Binbin Wu wrote:
> 
> 
> On 9/22/2023 4:33 AM, Sean Christopherson wrote:
> > Add an assertion that there are no in-progress MMU invalidations when a
> > VM is being destroyed, with the exception of the scenario where KVM
> > unregisters its MMU notifier between an .invalidate_range_start() call and
> > the corresponding .invalidate_range_end().
> > 
> > KVM can't detect unpaired calls from the mmu_notifier due to the above
> > exception waiver, but the assertion can detect KVM bugs, e.g. such as the
> > bug that *almost* escaped initial guest_memfd development.
> > 
> > Link: https://lore.kernel.org/all/e397d30c-c6af-e68f-d18e-b4e3739c5389@linux.intel.com
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   virt/kvm/kvm_main.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 54480655bcce..277afeedd670 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1381,9 +1381,16 @@ static void kvm_destroy_vm(struct kvm *kvm)
> >   	 * No threads can be waiting in kvm_swap_active_memslots() as the
> >   	 * last reference on KVM has been dropped, but freeing
> >   	 * memslots would deadlock without this manual intervention.
> > +	 *
> > +	 * If the count isn't unbalanced, i.e. KVM did NOT unregister between
> Nit: Readers can get it according to the code context, but is it better to
> add "MMU notifier"  to tell what to "unregister" to make the comment easier
> to understand?

Agreed, I'll add that when applying.