linux-kernel - Re: [PATCH 09/28] KVM: x86/mmu: Require mmu_lock be held for write in unyielding root iter

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Message-ID: <YZv7aIL8+7ESCfFc@google.com>
Date:   Mon, 22 Nov 2021 20:19:52 +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 09/28] KVM: x86/mmu: Require mmu_lock be held for write
 in unyielding root iter

On Mon, Nov 22, 2021, Ben Gardon wrote:
> > + * Holding mmu_lock for write obviates the need for RCU protection as the list
> > + * is guaranteed to be stable.
> > + */
> > +#define for_each_tdp_mmu_root(_kvm, _root, _as_id)                     \
> > +       list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)     \
> >                 if (kvm_mmu_page_as_id(_root) != _as_id) {              \
> > +                       lockdep_assert_held_write(&(_kvm)->mmu_lock);   \
> 
> Did you mean for this lockdep to only be hit in this uncommon
> non-matching ASID case?

Yes and no.  Yes, I intended what I wrote.  No, this isn't intended to be limited
to a memslot address space mismatch, but at the time I wrote this I was apparently
lazy or inept :-)

In hindsight, this would be better:

/* blah blah blah */
static inline struct list_head *kvm_get_tdp_mmu_roots_exclusive(struct kvm *kvm)
{
	lockdep_assert_held_write(&kvm->mmu_lock);

	return &kvm->arch.tdp_mmu_roots;
}

#define for_each_tdp_mmu_root(_kvm, _root, _as_id)			       \
	list_for_each_entry(_root, kvm_get_tdp_mmu_roots_exclusive(kvm), link) \
		if (kvm_mmu_page_as_id(_root) != _as_id) {		       \
		} else