[PATCH v2 15/15] KVM: Dynamically size memslot array based on number of used slots

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: "James Hogan" <jhogan@kernel.org>,
	"Paul Mackerras" <paulus@ozlabs.org>,
	"Christian Borntraeger" <borntraeger@de.ibm.com>,
	"Janosch Frank" <frankja@linux.ibm.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Marc Zyngier" <maz@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-mips@vger.kernel.org, kvm-ppc@vger.kernel.org,
	kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org
Subject: [PATCH v2 15/15] KVM: Dynamically size memslot array based on number of used slots
Date: Mon, 21 Oct 2019 17:35:37 -0700	[thread overview]
Message-ID: <20191022003537.13013-16-sean.j.christopherson@intel.com> (raw)
In-Reply-To: <20191022003537.13013-1-sean.j.christopherson@intel.com>

Now that the memslot logic doesn't assume memslots are always non-NULL,
dynamically size the array of memslots instead of unconditionally
allocating memory for the maximum number of memslots.

Note, because a to-be-deleted memslot must first be invalidated, the
array size cannot be immediately reduced when deleting a memslot.
However, consecutive deletions will realize the memory savings, i.e.
a second deletion will trim the entry.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 include/linux/kvm_host.h |  5 ++++-
 virt/kvm/kvm_main.c      | 31 ++++++++++++++++++++++++++++---
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3f8a7760bb79..9e3a68257e80 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -433,11 +433,14 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
  */
 struct kvm_memslots {
 	u64 generation;
-	struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
 	/* The mapping table from slot id to the index in memslots[]. */
 	short id_to_index[KVM_MEM_SLOTS_NUM];
 	atomic_t lru_slot;
 	int used_slots;
+	struct kvm_memory_slot memslots[];
+	/*
+	 * WARNING: 'memslots' is dynamically-sized.  It *MUST* be at the end.
+	 */
 };
 
 struct kvm {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 177caac395de..131b2dd7db72 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -535,7 +535,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
 		return NULL;
 
 	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
-		slots->id_to_index[i] = slots->memslots[i].id = -1;
+		slots->id_to_index[i] = -1;
 
 	return slots;
 }
@@ -934,6 +934,32 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
 	return old_memslots;
 }
 
+/*
+ * Note, at a minimum, the current number of used slots must be allocated, even
+ * when deleting a memslot, as we need a complete duplicate of the memslots for
+ * use when invalidating a memslot prior to deleting/moving the memslot.
+ */
+static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
+					     enum kvm_mr_change change)
+{
+	struct kvm_memslots *slots;
+	size_t old_size, new_size;
+
+	old_size = sizeof(struct kvm_memslots) +
+		   (sizeof(struct kvm_memory_slot) * old->used_slots);
+
+	if (change == KVM_MR_CREATE)
+		new_size = old_size + sizeof(struct kvm_memory_slot);
+	else
+		new_size = old_size;
+
+	slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
+	if (likely(slots))
+		memcpy(slots, old, old_size);
+
+	return slots;
+}
+
 static int kvm_set_memslot(struct kvm *kvm,
 			   const struct kvm_userspace_memory_region *mem,
 			   const struct kvm_memory_slot *old,
@@ -944,10 +970,9 @@ static int kvm_set_memslot(struct kvm *kvm,
 	struct kvm_memslots *slots;
 	int r;
 
-	slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
+	slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
 	if (!slots)
 		return -ENOMEM;
-	memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots));
 
 	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
 		/*
-- 
2.22.0

