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Using Shared Memory (NDIS 5.1)

Note   NDIS 5. x has been deprecated and is superseded by NDIS 6. x. For new NDIS driver development, see Network Drivers Starting with Windows Vista. For information about porting NDIS 5. x drivers to NDIS 6. x, see Porting NDIS 5.x Drivers to NDIS 6.0.

Miniport drivers for bus-master DMA devices must allocate shared memory for use by the NIC and the NIC driver. Special precautions are necessary when sharing cached memory between such a driver and its NIC. On certain architectures, specific steps must be taken to ensure that the memory is coherent because the NIC can access the shared physical memory directly, while the NIC driver accesses memory through the cache. This can cause differences in what the driver and the NIC detect in memory, even if they look at the same location.

The use of noncached shared memory avoids most of the problems associated with cached shared memory. However, noncached memory is a scarce system resource and allocation of such memory can be limited. When data written to physical memory must use a small write queue, even noncached data might not appear in physical memory immediately. Additionally, there are cases when using noncached memory is neither recommended nor possible. For example, data that the NIC driver reads repeatedly should always be cached to improve performance. Or, as another example, a NIC driver might be required to pass a received data packet to a number of different protocol drivers. Cached memory would be highly recommended in these cases. The NIC driver should also use cached memory for any data passed in by a protocol driver.

NdisMAllocateSharedMemorycan be called by a bus-master NIC driver to allocate memory for permanent sharing between the network adapter and the NIC driver. This function returns a virtual address and a physical address for the shared memory. The addresses are valid until a call to NdisMFreeSharedMemoryfrees the memory.

When shared cached memory is used, NDIS provides a function that must be called by the NIC driver to ensure coherency between what the NIC sees and what the NIC driver sees. Before accessing the shared memory on a send or on a receive, the NIC driver must call NdisFlushBufferand also (prior to Windows Vista) NdisMUpdateSharedMemoryto ensure cache coherence.



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