因为所有对象都是从8字节边界开始，并且大小可以被8整除，所以底部的3位不用于表示大小；我们将其作为标志，用于表示对象的不同状态。
As all objects start on an 8 byte boundary, and the size is divisible by 8, the bottom 3 bits are not used for the size; we use them for flags to indicate different states of the object.
这种方法使用可接受状态(共4种)的位掩码标志。
This method takes a bitmask flag of the acceptable states, of which there are four.
VPA提供了一个空闲标志，操作系统在空闲的时候将该标志置位，以便向Hypervisor说明其状态。
The VPA consists of an idle flag that the operating system sets when it is idle, to indicate the status to the hypervisor.
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