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

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Message-ID: <e9090079-2255-5a70-f909-89f6f65c12ed@redhat.com>
Date:   Mon, 3 May 2021 15:42:21 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     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 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.

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];

Paolo