kvm: introduce manual dirty log reprotect

There are two problems with KVM_GET_DIRTY_LOG.  First, and less important,
it can take kvm->mmu_lock for an extended period of time.  Second, its user
can actually see many false positives in some cases.  The latter is due
to a benign race like this:

  1. KVM_GET_DIRTY_LOG returns a set of dirty pages and write protects
     them.
  2. The guest modifies the pages, causing them to be marked ditry.
  3. Userspace actually copies the pages.
  4. KVM_GET_DIRTY_LOG returns those pages as dirty again, even though
     they were not written to since (3).

This is especially a problem for large guests, where the time between
(1) and (3) can be substantial.  This patch introduces a new
capability which, when enabled, makes KVM_GET_DIRTY_LOG not
write-protect the pages it returns.  Instead, userspace has to
explicitly clear the dirty log bits just before using the content
of the page.  The new KVM_CLEAR_DIRTY_LOG ioctl can also operate on a
64-page granularity rather than requiring to sync a full memslot;
this way, the mmu_lock is taken for small amounts of time, and
only a small amount of time will pass between write protection
of pages and the sending of their content.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/uapi/linux/kvm.h

@@ -492,6 +492,17 @@ struct kvm_dirty_log {
 	};
 };
 
+/* for KVM_CLEAR_DIRTY_LOG */
+struct kvm_clear_dirty_log {
+	__u32 slot;
+	__u32 num_pages;
+	__u64 first_page;
+	union {
+		void __user *dirty_bitmap; /* one bit per page */
+		__u64 padding2;
+	};
+};
+
 /* for KVM_SET_SIGNAL_MASK */
 struct kvm_signal_mask {
 	__u32 len;
@@ -975,6 +986,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_HYPERV_ENLIGHTENED_VMCS 163
 #define KVM_CAP_EXCEPTION_PAYLOAD 164
 #define KVM_CAP_ARM_VM_IPA_SIZE 165
+#define KVM_CAP_MANUAL_DIRTY_LOG_PROTECT 166
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
@@ -1421,6 +1433,9 @@ struct kvm_enc_region {
 #define KVM_GET_NESTED_STATE         _IOWR(KVMIO, 0xbe, struct kvm_nested_state)
 #define KVM_SET_NESTED_STATE         _IOW(KVMIO,  0xbf, struct kvm_nested_state)
 
+/* Available with KVM_CAP_MANUAL_DIRTY_LOG_PROTECT */
+#define KVM_CLEAR_DIRTY_LOG          _IOWR(KVMIO, 0xc0, struct kvm_clear_dirty_log)
+
 /* Secure Encrypted Virtualization command */
 enum sev_cmd_id {
 	/* Guest initialization commands */