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The info below is provided by one of Intel's experts on the 82567.
Please check out the Software developer’s manual at the link below:
Chapter 7.6 describes the interaction with the hardware, software, and Firmware semaphores. Table 65 describes the details of the Extended Configuration Control which has the SWFLAG bit.
If the SWFLAG bit is not sticking, it is likely that the semaphore is owned by either the hardware or the firmware. Please check if the Semaphore is already owned and if not then try writing the bit.
Let us know if this helps.