netdev - Re: [PATCH bpf-next v4 4/6] selftests/bpf: refactor xdp_context

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Message-ID: <67c08b96d1f88_37f929294af@willemb.c.googlers.com.notmuch>
Date: Thu, 27 Feb 2025 10:58:14 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Marcus Wichelmann <marcus.wichelmann@...zner-cloud.de>, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 bpf@...r.kernel.org, 
 linux-kselftest@...r.kernel.org
Cc: willemdebruijn.kernel@...il.com, 
 jasowang@...hat.com, 
 andrew+netdev@...n.ch, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
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 daniel@...earbox.net, 
 andrii@...nel.org, 
 martin.lau@...ux.dev, 
 eddyz87@...il.com, 
 song@...nel.org, 
 yonghong.song@...ux.dev, 
 john.fastabend@...il.com, 
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 mykolal@...com, 
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 hawk@...nel.org, 
 marcus.wichelmann@...zner-cloud.de
Subject: Re: [PATCH bpf-next v4 4/6] selftests/bpf: refactor
 xdp_context_functional test and bpf program

Marcus Wichelmann wrote:
> The existing XDP metadata test works by creating a veth pair and
> attaching XDP & TC programs that drop the packet when the condition of
> the test isn't fulfilled. The test then pings through the veth pair and
> succeeds when the ping comes through.
> 
> While this test works great for a veth pair, it is hard to replicate for
> tap devices to test the XDP metadata support of them. A similar test for
> the tun driver would either involve logic to reply to the ping request,
> or would have to capture the packet to check if it was dropped or not.
> 
> To make the testing of other drivers easier while still maximizing code
> reuse, this commit refactors the existing xdp_context_functional test to
> use a test_result map. Instead of conditionally passing or dropping the
> packet, the TC program is changed to copy the received metadata into the
> value of that single-entry array map. Tests can then verify that the map
> value matches the expectation.
> 
> This testing logic is easy to adapt to other network drivers as the only
> remaining requirement is that there is some way to send a custom
> Ethernet packet through it that triggers the XDP & TC programs.
> 
> The Ethernet header of that custom packet is all-zero, because it is not
> required to be valid for the test to work. The zero ethertype also helps
> to filter out packets that are not related to the test and would
> otherwise interfere with it.
> 
> The payload of the Ethernet packet is used as the test data that is
> expected to be passed as metadata from the XDP to the TC program and
> written to the map. It has a fixed size of 32 bytes which is a
> reasonable size that should be supported by both drivers. Additional
> packet headers are not necessary for the test and were therefore skipped
> to keep the testing code short.
> 
> This new testing methodology no longer requires the veth interfaces to
> have IP addresses assigned, therefore these were removed.
> 
> Signed-off-by: Marcus Wichelmann <marcus.wichelmann@...zner-cloud.de>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>