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

From: Paolo Bonzini <pbonzini@redhat.com>
To: Ben Gardon <bgardon@google.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: Peter Xu <peterx@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Peter Shier <pshier@google.com>,
	Junaid Shahid <junaids@google.com>,
	Jim Mattson <jmattson@google.com>,
	Yulei Zhang <yulei.kernel@gmail.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active
Date: Mon, 3 May 2021 15:42:21 +0200	[thread overview]
Message-ID: <e9090079-2255-5a70-f909-89f6f65c12ed@redhat.com> (raw)
In-Reply-To: <20210429211833.3361994-2-bgardon@google.com>

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