PRB: Performance Degradation When You Use the NDIS.H Header File (312329)

CAUSE

This behavior may occur because the macro definition of NdisQueryBufferSafe in the Windows 2000 DDK NDIS.H header file is incorrect. This macro is defined as follows:

#define NdisQueryBufferSafe(_Buffer, _VirtualAddress, _Length,
_Priority) \ 
{ \ 
PVOID _VA; \ 
\ 
_VA = MmGetSystemAddressForMdlSafe(_Buffer, _Priority); \ 
if (ARGUMENT_PRESENT(_VirtualAddress)) \ 
{ \ 
*(PVOID *)(_VirtualAddress) = _VA; \ 
} \ 
*(_Length) = (_VA != NULL) ? MmGetMdlByteCount(_Buffer) : 0; \ 
}
				

This macro is properly used only when you want to build non-binary-compatible (!BINARY_COMPATIBLE) NDIS drivers. The problem in the preceding definition of the macro is that MmGetSystemAddressForMdlSafe is called unconditionally, regardless of whether the _VirtualAddress argument is present or not. The driver passes a _VirtualAddress argument only when it has to copy user data (for example, to coalesce the whole packet or just the first 54 bytes for the MAC, IP, or TCP header). Therefore, by unconditionally calling MmGetSystemAddressForMdlSafe, performance degradation may result.

#define NdisQueryBufferSafe(_Buffer, _VirtualAddress, _Length, _Priority)\ 
{\ 
    if (ARGUMENT_PRESENT(_VirtualAddress))\ 
    {\ 
        *(PVOID *)(_VirtualAddress) = MmGetSystemAddressForMdlSafe(_Buffer,_Priority); \ 
    }\ 
    *(_Length) = MmGetMdlByteCount(_Buffer);\ 
}