From: Stephen Hemminger <stephen@networkplumber.org>
To: Joshua Washington <joshwash@google.com>
Cc: Junfeng Guo <junfeng.guo@intel.com>,
Jeroen de Borst <jeroendb@google.com>,
Rushil Gupta <rushilg@google.com>,
dev@dpdk.org, Ferruh Yigit <ferruh.yigit@amd.com>
Subject: Re: [PATCH] net/gve: Update max_rx_pktlen to be based on MTU
Date: Mon, 16 Oct 2023 15:58:51 -0700 [thread overview]
Message-ID: <20231016155851.32978666@hermes.local> (raw)
In-Reply-To: <20231016205948.2252342-1-joshwash@google.com>
On Mon, 16 Oct 2023 13:59:48 -0700
Joshua Washington <joshwash@google.com> wrote:
> conjunction with the MTU fix, causes problems with testpmd, as setting the
> packet length with the --max-pkt-len flag causes the MTU to be set
> higher than possible due to underflow.
>
> As an example, setting --max-pkt-len=1460 (the default MTU on Google
> Cloud VMs) causes testpmd to set the following:
> mtu = 1460 - eth_overhead,
>
> where eth_overhead = dev->max_rx_pktlen - dev->max_mtu = 65535 - 1460.
>
> Thus, mtu = 1460 - 65535 + 1460 = 2921 due to underflow.
>
> Signed-off-by: Joshua Washington <joshwash@google.com>
>
> Fixes: 030025b74202 ("net/gve: fix max MTU limit")
> Cc: joshwash@google.com
Never mind, previous comment.
You are correctly doing the inverse of this common code pattern.
In testpmd:
static uint32_t
eth_dev_get_overhead_len(uint32_t max_rx_pktlen, uint16_t max_mtu)
{
uint32_t overhead_len;
if (max_mtu != UINT16_MAX && max_rx_pktlen > max_mtu)
overhead_len = max_rx_pktlen - max_mtu;
else
overhead_len = RTE_ETHER_HDR_LEN + RTE_ETHER_CRC_LEN;
return overhead_len;
}
next prev parent reply other threads:[~2023-10-16 22:58 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-16 20:59 Joshua Washington
2023-10-16 21:20 ` Stephen Hemminger
2023-10-16 22:58 ` Stephen Hemminger [this message]
2023-10-26 17:38 ` Ferruh Yigit
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