KVM: x86: add SYNC_REGS_SIZE_BYTES #define.

Replace hardcoded padding size value for struct kvm_sync_regs
with #define SYNC_REGS_SIZE_BYTES.

Also update the value specified in api.txt from outdated hardcoded
value to SYNC_REGS_SIZE_BYTES.

Signed-off-by: Ken Hofsass <hofsass@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -3905,7 +3905,7 @@ in userspace.
 	__u64 kvm_dirty_regs;
 	union {
 		struct kvm_sync_regs regs;
-		char padding[1024];
+		char padding[SYNC_REGS_SIZE_BYTES];
 	} s;
 
 If KVM_CAP_SYNC_REGS is defined, these fields allow userspace to access

include/uapi/linux/kvm.h

@@ -396,6 +396,10 @@ struct kvm_run {
 		char padding[256];
 	};
 
+	/* 2048 is the size of the char array used to bound/pad the size
+	 * of the union that holds sync regs.
+	 */
+	#define SYNC_REGS_SIZE_BYTES 2048
 	/*
 	 * shared registers between kvm and userspace.
 	 * kvm_valid_regs specifies the register classes set by the host
@@ -407,7 +411,7 @@ struct kvm_run {
 	__u64 kvm_dirty_regs;
 	union {
 		struct kvm_sync_regs regs;
-		char padding[2048];
+		char padding[SYNC_REGS_SIZE_BYTES];
 	} s;
 };