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<div class="moz-cite-prefix">On 15-07-2026 22:30, Maxime Leroy
wrote:<br>
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<blockquote type="cite" cite="mid:CAHHRULVvBsUjxVJh++BKsV_tGJLnh2Fuo+fJYh-=M_rsTKmVtg@mail.gmail.com">
<pre wrap="" class="moz-quote-pre">Hi Hemant,
Do you have any preference between the different approaches proposed below?
Regards,
Maxime Leroy
On Tue, Jun 30, 2026 at 2:39 PM Maxime Leroy <a class="moz-txt-link-rfc2396E" href="mailto:maxime@leroys.fr"><maxime@leroys.fr></a> wrote:
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<pre wrap="" class="moz-quote-pre">
When RTE_ETH_RSS_LEVEL_INNERMOST is requested, the IP key extracts use
the innermost header index (HDR_INDEX_LAST). The hardware only resolves
that index when several IP headers are stacked: for a non-tunnelled
frame, which carries a single IP header, the extraction returns nothing.
The RSS hash is then constant and all such frames are steered to a
single Rx queue.
Always also extract the outer IP (header index 0), which the hardware
resolves for any frame. Non-tunnelled frames are thus hashed on their
only IP header, while tunnelled frames keep being hashed on their inner
IP.
This is a deliberate tradeoff: the ethdev API defines
RTE_ETH_RSS_LEVEL_INNERMOST as hashing the innermost header only, but the
hardware cannot do that without breaking RSS for plain traffic. As a
consequence, two tunnelled flows with the same inner header but
different outer IPs may hash to different queues. This limitation is
documented in the dpaa2 guide.
Alternatives considered (feedback welcome, hence RFC):
- Hash both outer and inner only under RTE_ETH_RSS_LEVEL_PMD_DEFAULT and
keep INNERMOST strictly inner-only. The ethdev API leaves the default
level to the PMD, so this stays API-compliant; it changes the default
hash for tunnelled traffic.
- Add a generic RTE_ETH_RSS_LEVEL_OUTER_INNER value to the ethdev API so
applications can request hashing on both encapsulation levels
explicitly, instead of overloading INNERMOST. This needs an ethdev API
change and agreement from other PMDs.
Fixes: 32f701671d2f ("net/dpaa2: support inner RSS level for tunnelled traffic")
Signed-off-by: Maxime Leroy <a class="moz-txt-link-rfc2396E" href="mailto:maxime@leroys.fr"><maxime@leroys.fr></a></pre>
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<div style="font-family:'Segoe UI';font-size:14px;font-style:normal;font-weight:400;line-height:20px">
<p>Hi Maxime,</p>
<p>My preference is to keep <code>RTE_ETH_RSS_LEVEL_INNERMOST</code>
semantics unchanged and strictly hash on the innermost header.</p>
<p>I would rather use the PMD-defined default level to provide the
best RSS distribution for both plain and tunnelled traffic.
Overloading <code>INNERMOST</code> to include the outer IP
makes the behavior diverge from the ethdev definition and may
surprise applications relying on true inner-header affinity.</p>
<p>Longer term, an explicit <code>OUTER_INNER</code> RSS level
would be the cleanest solution if there is interest from other
PMDs.</p>
<p>Regards, </p>
<p>Hemant</p>
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