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Date: Fri, 15 Dec 2023 09:21:12 -0800
From: Jakub Kicinski <kuba@kernel.org>
To: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Cc: Michael Chan <michael.chan@broadcom.com>, davem@davemloft.net,
 netdev@vger.kernel.org, edumazet@google.com, pabeni@redhat.com,
 bpf@vger.kernel.org, hawk@kernel.org, ast@kernel.org, daniel@iogearbox.net,
 john.fastabend@gmail.com, Somnath Kotur <somnath.kotur@broadcom.com>
Subject: Re: [PATCH net] bnxt_en: do not map packet buffers twice
Message-ID: <20231215092112.3f0fee3d@kernel.org>
In-Reply-To: <ZXyFW0lIGluM8ipj@C02YVCJELVCG.dhcp.broadcom.net>
References: <20231214213138.98095-1-michael.chan@broadcom.com>
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On Fri, 15 Dec 2023 11:57:14 -0500 Andy Gospodarek wrote:
> > This patch is all good, but I'm confused by the handling of head.
> > Do you recycle it immediately and hope that the Tx happens before
> > the Rx gets around to using the recycled page again? Am I misreading?  
> 
> Your description is correct, but we use a better strategy that just
> hoping it works out. :)
> 
> The design is that we do not update the rx ring with the producer value
> that was present when the packet was received until after getting the tx
> completion indicating that the packet sent via XDP_TX action has been
> sent.

Ah, I see it, interesting! In that case - next question.. :)

Are the XDP_REDIRECT (target) and XDP_TX going to the same rings?
The locking seems to be missing, and bnxt_tx_int_xdp() does not
seem to be able to handle the optimization you described if
a ring contains a mix of XDP_REDIRECT and XDP_TX.

If I'm reading the assignment in bnxt_alloc_mem() and indexing
right - XDP_REDIRECT and XDP_TX do seem to go to the same rings.