linux-kernel - Re: [PATCH 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for defunct TDP MMU root

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Message-ID: <YbkuM4jEbJX6QQR6@google.com>
Date:   Tue, 14 Dec 2021 23:52:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>
Subject: Re: [PATCH 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for
 defunct TDP MMU root

On Tue, Dec 14, 2021, Sean Christopherson wrote:
> Assuming there is exactly one helper, that would also address my
> concerns with kvm_tdp_mmu_zap_invalidated_roots() being unsafe to call in parallel,
> e.g. two zappers processing an invalid root would both put the last reference to
> a root and trigger use-after-free of a different kind.

I take that back.  So long as both callers grabbed a reference to the root, multiple
instances are ok.  I forgot that kvm_tdp_mmu_zap_invalidated_roots() doesn't take
roots off the list directly, that's handled by kvm_tdp_mmu_put_root().