On Thu, 17 Oct 2019 07:27:34 +0000
Shahaf Shuler [off-list ref] wrote:

Some PMDs inline the mbuf data buffer directly to device. This is in
order to save the overhead of the PCI headers involved when the device
DMA read the buffer pointer. For some devices it is essential in order
to reach the pick BW.

However, there are cases where such inlining is in-efficient. For example
when the data buffer resides on other device memory (like GPU or storage
device). attempt to inline such buffer will result in high PCI overhead
for reading and copying the data from the remote device.

To support a mixed traffic pattern (some buffers from local DRAM, some
buffers from other devices) with high BW, a hint flag is introduced in
the mbuf.
Application will hint the PMD whether or not it should try to inline the
given mbuf data buffer. PMD should do best effort to act upon this
request.

Signed-off-by: Shahaf Shuler <redacted>

This kind of optimization is hard, and pushing the problem to the application
to decide seems like the wrong step. Can the driver just infer this
already because some mbuf's are external?