Re: [PATCH net-next 1/4] net: Introduce NETIF_F_GRO_HW

From: Alexander Duyck <redacted>
Date: Mon, 4 Dec 2017 10:43:58 -0800

On Mon, Dec 4, 2017 at 10:23 AM, Michael Chan [off-list ref] wrote:

quoted

On Mon, Dec 4, 2017 at 8:47 AM, Alexander Duyck
[off-list ref] wrote:

quoted

On Mon, Dec 4, 2017 at 3:12 AM, Michael Chan [off-list ref] wrote:

quoted

Introduce NETIF_F_GRO_HW feature flag for NICs that support hardware
GRO.  With this flag, we can now independently turn on or off hardware
GRO when GRO is on.  Hardware GRO guarantees that packets can be
re-segmented by TSO/GSO to reconstruct the original packet stream.

Cc: Ariel Elior <redacted>
Cc: <redacted>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>

Do we really need yet another feature bit for this? We already have
LRO and GRO and now we have to add something that isn't quite either
one?

I think so, to be consistent with TSO/GSO on the transmit side.  On
the receive side, we have LRO/GRO_HW/GRO.  There is difference between
LRO/GRO_HW that we need to distinguish between the 2.

I don't really see the difference. Your GRO_HW likely doens't do all
of the stuff GRO can do. Neither does LRO. Both occur in the hardware
normally. It would make sense to reuse the LRO flag for this instead
of coming up with a new feature flag that makes things confusing by
saying you are doing a software offload in hardware.

I view LRO as a subset of what GRO can handle, that is performed in
hardware. From the stack perspective the only thing that really
matters is that the frames can be segmented back into what they were
before they were assembled. That is why I think it would be better to
add a flag indicating that the LRO is reversible instead of adding yet
another feature bit that the user has to toggle. That way if at some
point in the future an issue is found where your "GRO in hardware"
feature has a bug that isn't reversible it is just a matter of
clearing the privage flag bit and the mechanisms already in place for
dealing with assembly and routing can take over.

I don't think they should use the LRO flag.

If their HW GRO stream is fully reversible, which it is, then it's not
LRO.

LRO gets disabled when bridging or routing is enabled, and HW GRO
should not take this penalty.