[ovs-dev] [PATCH v3] datapath-windows: Add GRE checksum

[Nithin Raju]

> #endif
>@@ -299,34 +312,21 @@ OvsDecapGre(POVS_SWITCH_CONTEXT switchContext,
>     EthHdr *ethHdr;
>     IPHdr *ipHdr;
>     GREHdr *greHdr;
>-    UINT32 tunnelSize = 0, packetLength = 0;
>+    UINT32 tunnelSize, packetLength;
>     UINT32 headRoom = 0;
>     PUINT8 bufferStart;
>     NDIS_STATUS status;
> 
>     curNb = NET_BUFFER_LIST_FIRST_NB(curNbl);
>     packetLength = NET_BUFFER_DATA_LENGTH(curNb);
>-    tunnelSize = GreTunHdrSize(tunKey->flags);
>+    curMdl = NET_BUFFER_CURRENT_MDL(curNb);
>+    tunnelSize = GreTunHdrSize(0);
>     if (packetLength <= tunnelSize) {
>         return NDIS_STATUS_INVALID_LENGTH;
>     }
> 
>-    /*
>-     * Create a copy of the NBL so that we have all the headers in one
>MDL.
>-     */
>-    *newNbl = OvsPartialCopyNBL(switchContext, curNbl,
>-                                tunnelSize + OVS_DEFAULT_COPY_SIZE, 0,
>-                                TRUE /*copy NBL info */);
>-
>-    if (*newNbl == NULL) {
>-        return NDIS_STATUS_RESOURCES;
>-    }
>-
>-    curNbl = *newNbl;
>-    curNb = NET_BUFFER_LIST_FIRST_NB(curNbl);
>-    curMdl = NET_BUFFER_CURRENT_MDL(curNb);
>-    bufferStart = (PUINT8)MmGetSystemAddressForMdlSafe(curMdl,
>LowPagePriority) +
>-                  NET_BUFFER_CURRENT_MDL_OFFSET(curNb);
>+    /* Get a contignuous buffer for the maxmimum length of a GRE header
>*/
>+    bufferStart = NdisGetDataBuffer(curNb, OVS_MAX_GRE_LGTH, NULL, 1, 0);

Sorry we have to go back and forth on this. Like I mentioned in the
previous email, the idea is to get a contiguous chunk of memory so we can
walk all the header until the GRE header. It is a good idea to use
NdisGetDataBuffer() instead of copying the NBL. But, we won¹t avoid the
copied NBL anyway since decap has to happen on the copied NBL.

In any case, NdisGetDataBuffer() has some pitfalls:
"If the requested data in the buffer is contiguous, this return value is a
pointer to a location that NDIS provides. If the data is not contiguous,
NDIS uses the Storage parameter as follows:

* If the Storage parameter is non-NULL, NDIS copies the data to the buffer
at Storage. This return value is the pointer passed to the Storage
parameter.
* If the Storage parameter is NULL, this return value is NULL.²

So, if the first MDL does not fit the headers until the GRE headers, we
need to pass explicit memory to NdisGetDataBuffer() in argument #3 in
order for NDIS to copy it over to a contiguous chunk of memory.

Thanks,
-- Nithin