KVM: x86/mmu: Protect the tdp_mmu_roots list with RCU

Protect the contents of the TDP MMU roots list with RCU in preparation
for a future patch which will allow the iterator macro to be used under
the MMU lock in read mode.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-9-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 16 insertions, 5 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0d217c3db3f8..2f6f4aeca6b7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1059,25 +1059,36 @@ struct kvm_arch {
 	bool tdp_mmu_enabled;
 
 	/*
-	 * List of struct kvmp_mmu_pages being used as roots.
+	 * List of struct kvm_mmu_pages being used as roots.
 	 * All struct kvm_mmu_pages in the list should have
 	 * tdp_mmu_page set.
-	 * All struct kvm_mmu_pages in the list should have a positive
-	 * root_count except when a thread holds the MMU lock and is removing
-	 * an entry from the list.
+	 *
+	 * For reads, this list is protected by:
+	 *	the MMU lock in read mode + RCU or
+	 *	the MMU lock in write mode
+	 *
+	 * For writes, this list is protected by:
+	 *	the MMU lock in read mode + the tdp_mmu_pages_lock or
+	 *	the MMU lock in write mode
+	 *
+	 * Roots will remain in the list until their tdp_mmu_root_count
+	 * drops to zero, at which point the thread that decremented the
+	 * count to zero should removed the root from the list and clean
+	 * it up, freeing the root after an RCU grace period.
 	 */
 	struct list_head tdp_mmu_roots;
 
 	/*
 	 * List of struct kvmp_mmu_pages not being used as roots.
 	 * All struct kvm_mmu_pages in the list should have
-	 * tdp_mmu_page set and a root_count of 0.
+	 * tdp_mmu_page set and a tdp_mmu_root_count of 0.
 	 */
 	struct list_head tdp_mmu_pages;
 
 	/*
 	 * Protects accesses to the following fields when the MMU lock
 	 * is held in read mode:
+	 *  - tdp_mmu_roots (above)
 	 *  - tdp_mmu_pages (above)
 	 *  - the link field of struct kvm_mmu_pages used by the TDP MMU
 	 *  - lpage_disallowed_mmu_pages