KVM: Make dirty ring exclusive to dirty bitmap log

There's no good reason to use both the dirty bitmap logging and the
new dirty ring buffer to track dirty bits.  We should be able to even
support both of them at the same time, but it could complicate things
which could actually help little.  Let's simply make it the rule
before we enable dirty ring on any arch, that we don't allow these two
interfaces to be used together.

The big world switch would be KVM_CAP_DIRTY_LOG_RING capability
enablement.  That's where we'll switch from the default dirty logging
way to the dirty ring way.  As long as kvm->dirty_ring_size is setup
correctly, we'll once and for all switch to the dirty ring buffer mode
for the current virtual machine.
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201001012224.5818-1-peterx@redhat.com>
[Change errno from EINVAL to ENXIO. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -6489,3 +6489,10 @@ processor's dirty page buffers into the kernel buffer (with dirty bitmaps, the
 flushing is done by the KVM_GET_DIRTY_LOG ioctl).  To achieve that, one
 needs to kick the vcpu out of KVM_RUN using a signal.  The resulting
 vmexit ensures that all dirty GFNs are flushed to the dirty rings.
+
+NOTE: the capability KVM_CAP_DIRTY_LOG_RING and the corresponding
+ioctl KVM_RESET_DIRTY_RINGS are mutual exclusive to the existing ioctls
+KVM_GET_DIRTY_LOG and KVM_CLEAR_DIRTY_LOG.  After enabling
+KVM_CAP_DIRTY_LOG_RING with an acceptable dirty ring size, the virtual
+machine will switch to ring-buffer dirty page tracking and further
+KVM_GET_DIRTY_LOG or KVM_CLEAR_DIRTY_LOG ioctls will fail.

virt/kvm/kvm_main.c

@@ -1426,6 +1426,10 @@ int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
 	unsigned long n;
 	unsigned long any = 0;
 
+	/* Dirty ring tracking is exclusive to dirty log tracking */
+	if (kvm->dirty_ring_size)
+		return -ENXIO;
+
 	*memslot = NULL;
 	*is_dirty = 0;
 
@@ -1487,6 +1491,10 @@ static int kvm_get_dirty_log_protect(struct kvm *kvm, struct kvm_dirty_log *log)
 	unsigned long *dirty_bitmap_buffer;
 	bool flush;
 
+	/* Dirty ring tracking is exclusive to dirty log tracking */
+	if (kvm->dirty_ring_size)
+		return -ENXIO;
+
 	as_id = log->slot >> 16;
 	id = (u16)log->slot;
 	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
@@ -1595,6 +1603,10 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
 	unsigned long *dirty_bitmap_buffer;
 	bool flush;
 
+	/* Dirty ring tracking is exclusive to dirty log tracking */
+	if (kvm->dirty_ring_size)
+		return -ENXIO;
+
 	as_id = log->slot >> 16;
 	id = (u16)log->slot;
 	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)