linux-kernel - Re: [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active

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Message-ID: <CANgfPd9O3d9b+WYgo+ke1Jx50=ep_f-ZC1gRqUET6PDsLxW+Gw@mail.gmail.com>
Date:   Tue, 4 May 2021 10:26:03 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active

On Mon, May 3, 2021 at 6:42 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 29/04/21 23:18, Ben Gardon wrote:
> > +void activate_shadow_mmu(struct kvm *kvm)
> > +{
> > +     kvm->arch.shadow_mmu_active = true;
> > +}
> > +
>
> I think there's no lock protecting both the write and the read side.
> Therefore this should be an smp_store_release, and all checks in
> patch 2 should be an smp_load_acquire.

That makes sense.

>
> Also, the assignments to slot->arch.rmap in patch 4 (alloc_memslot_rmap)
> should be an rcu_assign_pointer, while __gfn_to_rmap must be changed like so:
>
> +       struct kvm_rmap_head *head;
> ...
> -       return &slot->arch.rmap[level - PG_LEVEL_4K][idx];
> +       head = srcu_dereference(slot->arch.rmap[level - PG_LEVEL_4K], &kvm->srcu,
> +                                lockdep_is_held(&kvm->slots_arch_lock));
> +       return &head[idx];

I'm not sure I fully understand why this becomes necessary after patch
4. Isn't it already needed since the memslots are protected by RCU? Or
is there already a higher level rcu dereference?

__kvm_memslots already does an srcu dereference, so is there a path
where we aren't getting the slots from that function where this is
needed?

I wouldn't say that the rmaps are protected by RCU in any way that
separate from the memslots.

>
> Paolo
>