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

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Date:   Mon, 24 Apr 2023 17:36:37 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2] KVM: x86: Preserve TDP MMU roots until they are
 explicitly invalidated

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 :-/