On Thu, Feb 06, 2020 at 05:12:08PM -0500, Peter Xu wrote:

On Tue, Jan 21, 2020 at 02:31:56PM -0800, Sean Christopherson wrote:

quoted

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.

Tested-by: Christoffer Dall <redacted>
Tested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Sean Christopherson <redacted>
---
 include/linux/kvm_host.h |  2 +-
 virt/kvm/kvm_main.c      | 31 ++++++++++++++++++++++++++++---
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 60ddfdb69378..8bb6fb127387 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h

@@ -431,11 +431,11 @@ 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[];

This patch is tested so I believe this works, however normally I need
to do similar thing with [0] otherwise gcc might complaint.  Is there
any trick behind to make this work?  Or is that because of different
gcc versions?

array[] and array[0] have the same net affect, but array[] is given special
treatment by gcc to provide extra sanity checks, e.g. requires the field to
be the end of the struct.  Last I checked, gcc also doesn't allow array[]
in unions.  There are probably other restrictions.

But, it's precisely because of those restrictions that using array[] is
preferred, as it provides extra protections, e.g. if someone moved memslots
to the top of the struct it would fail to compile.
 

quoted

 };
 
 struct kvm {

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9b614cf2ca20..ed392ce64e59 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -565,7 +565,7 @@ 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;
 }

@@ -1077,6 +1077,32 @@ 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);

(Maybe directly copy into it?)

I don't follow, are you saying do "*slots = *old"?

@new_size and @old_size are not guaranteed to be the same.  More
specifically, slots->memslots and old->slots are now flexible arrays with
potentially different sizes.  Doing "*slots = *old" would only copy the
standard members, a memcpy() would still be needed for @memlots.

A more effecient implementation would be:

	slots = kvalloc(new_size, GFP_KERNEL_ACCOUNT);
	if (likely(slots)) {
		memcpy(slots, old, old_size);
		if (change == KVM_MR_CREATE)
			memset((void *)slots + old_size, 0, new_size - old_size);
	}

to avoid unnecessarily zeroing out the entire thing.  I opted for the
simpler implementation as this is not performance critical code, for most
cases @slots won't be all that large, and I wanted to be absolutely sure
any mixup would hit zeroed memory and not uninitialized memory.

quoted

+
+	return slots;
+}
+
 static int kvm_set_memslot(struct kvm *kvm,
 			   const struct kvm_userspace_memory_region *mem,
 			   struct kvm_memory_slot *old,

@@ -1087,10 +1113,9 @@ 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) {
 		/*

-- 
2.24.1

-- 
Peter Xu

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