Re: [PATCH v2] KVM: x86: Preserve TDP MMU roots until they are explicitly invalidated

[Sean Christopherson]

On Tue, Apr 25, 2023, David Matlack wrote:
> On Mon, Apr 24, 2023 at 05:36:37PM -0700, Sean Christopherson wrote:
> > On Mon, Apr 24, 2023, David Matlack wrote:
> > > It'd be nice to keep around the lockdep assertion though for the other (and
> > > future) callers. The cleanest options I can think of are:
> > > 
> > > 1. Pass in a bool "vm_teardown" kvm_tdp_mmu_invalidate_all_roots() and
> > > use that to gate the lockdep assertion.
> > > 2. Take the mmu_lock for read in kvm_mmu_uninit_tdp_mmu() and pass
> > > down bool shared to kvm_tdp_mmu_invalidate_all_roots().
> > > 
> > > Both would satisfy your concern of not blocking teardown on the async
> > > worker and my concern of keeping the lockdep check. I think I prefer
> > > (1) since, as you point out, taking the mmu_lock at all is
> > > unnecessary.
> > 
> > Hmm, another option:
> > 
> >  3. Refactor the code so that kvm_arch_init_vm() doesn't call
> >     kvm_tdp_mmu_invalidate_all_roots() when VM creation fails, and then lockdep
> >     can ignore on users_count==0 without hitting the false positive.
> > 
> > I like (2) the least.  Not sure I prefer (1) versus (3).  I dislike passing bools
> > just to ignore lockdep, but reworking code for a "never hit in practice" edge case
> > is arguably worse :-/
> 
> Agree (2) is the worst option. (3) seems potentially brittle (likely to
> trigger a false-positive lockdep warning if the code ever gets
> refactored back).
> 
> How about throwing some underscores at the problem?

LOL, now we're speaking my language.

I think I have a better option though.  The false positives on users_count can be
suppressed by gating the assert on kvm->created_vcpus.  If KVM_CREATE_VM fails then
it's impossible for the VM to have created vCPUs.  I like this option in particular
because it captures why it's safe for the KVM_CREATE_VM error path to run without
mmu_lock (no vCPUs == no roots).

I'll manually test this against the error path tomorrow:

	if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
	    refcount_read(&kvm->users_count) && kvm->created_vcpus)
		lockdep_assert_held_write(&kvm->mmu_lock);