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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	zy@yusur.tech, Hao Chen <chenh@yusur.tech>,
	"open list:NETWORKING DRIVERS" <netdev@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:VIRTIO CORE AND NET DRIVERS"
	<virtualization@lists.linux-foundation.org>,
	Eric Dumazet <edumazet@google.com>,
	hengqi@linux.alibaba.com, huangml@yusur.tech,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500
Date: Mon, 8 May 2023 14:10:07 -0400	[thread overview]
Message-ID: <20230508140640-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20230508092548.5fc8f078@hermes.local>

On Mon, May 08, 2023 at 09:25:48AM -0700, Stephen Hemminger wrote:
> On Mon, 8 May 2023 06:30:07 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > > > > I don't know, in any scenario, when the hardware supports a large mtu, but we do
> > > > > not want the user to use it by default.  
> > > >
> > > > When other devices on the same LAN have mtu set to 1500 and
> > > > won't accept bigger packets.  
> > > 
> > > So, that depends on pmtu/tcp-probe-mtu.
> > > 
> > > If the os without pmtu/tcp-probe-mtu has a bigger mtu, then it's big packet
> > > will lost.
> > > 
> > > Thanks.
> > >   
> > 
> > pmtu is designed for routing. LAN is supposed to be configured with
> > a consistent MTU.
> 
> Virtio is often used with bridging or macvlan which can't support PMTU.
> PMTU only works when forwarding at layer 3 (ie routing) where there is
> a IP address to send the ICMP response. If doing L2 forwarding, the
> only thin the bridge can do is drop the packet.
> 
> TCP cab recover but detecting an MTU blackhole requires retransmissions.

Exactly. That's why we basically use the MTU advice supplied by device
by default - it's designed for use-cases of software devices where
the device might have more information about the MTU than the guest.
If hardware devices want e.g. a way to communicate support for
jumbo frames without communicating any information about the LAN,
a new feature will be needed.

-- 
MST

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

WARNING: multiple messages have this Message-ID (diff)
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	huangml@yusur.tech, zy@yusur.tech,
	Jason Wang <jasowang@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	"open list:VIRTIO CORE AND NET DRIVERS" 
	<virtualization@lists.linux-foundation.org>,
	"open list:NETWORKING DRIVERS" <netdev@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	Hao Chen <chenh@yusur.tech>,
	hengqi@linux.alibaba.com
Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500
Date: Mon, 8 May 2023 14:10:07 -0400	[thread overview]
Message-ID: <20230508140640-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20230508092548.5fc8f078@hermes.local>

On Mon, May 08, 2023 at 09:25:48AM -0700, Stephen Hemminger wrote:
> On Mon, 8 May 2023 06:30:07 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > > > > I don't know, in any scenario, when the hardware supports a large mtu, but we do
> > > > > not want the user to use it by default.  
> > > >
> > > > When other devices on the same LAN have mtu set to 1500 and
> > > > won't accept bigger packets.  
> > > 
> > > So, that depends on pmtu/tcp-probe-mtu.
> > > 
> > > If the os without pmtu/tcp-probe-mtu has a bigger mtu, then it's big packet
> > > will lost.
> > > 
> > > Thanks.
> > >   
> > 
> > pmtu is designed for routing. LAN is supposed to be configured with
> > a consistent MTU.
> 
> Virtio is often used with bridging or macvlan which can't support PMTU.
> PMTU only works when forwarding at layer 3 (ie routing) where there is
> a IP address to send the ICMP response. If doing L2 forwarding, the
> only thin the bridge can do is drop the packet.
> 
> TCP cab recover but detecting an MTU blackhole requires retransmissions.

Exactly. That's why we basically use the MTU advice supplied by device
by default - it's designed for use-cases of software devices where
the device might have more information about the MTU than the guest.
If hardware devices want e.g. a way to communicate support for
jumbo frames without communicating any information about the LAN,
a new feature will be needed.

-- 
MST


  reply	other threads:[~2023-05-08 18:10 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-06  2:15 [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500 Hao Chen
2023-05-06  2:50 ` Xuan Zhuo
2023-05-06  2:50   ` Xuan Zhuo
2023-05-06  8:56   ` Hao Chen
2023-05-07  8:58     ` Michael S. Tsirkin
2023-05-07  8:58       ` Michael S. Tsirkin
2023-05-08  2:01       ` Xuan Zhuo
2023-05-08  2:01         ` Xuan Zhuo
2023-05-08  6:15         ` Michael S. Tsirkin
2023-05-08  6:15           ` Michael S. Tsirkin
2023-05-08  6:18           ` Xuan Zhuo
2023-05-08  6:18             ` Xuan Zhuo
2023-05-08  6:43             ` Michael S. Tsirkin
2023-05-08  6:43               ` Michael S. Tsirkin
2023-05-08  7:41               ` Xuan Zhuo
2023-05-08  7:41                 ` Xuan Zhuo
2023-05-08 10:30                 ` Michael S. Tsirkin
2023-05-08 10:30                   ` Michael S. Tsirkin
2023-05-08 16:25                   ` Stephen Hemminger
2023-05-08 18:10                     ` Michael S. Tsirkin [this message]
2023-05-08 18:10                       ` Michael S. Tsirkin
2023-05-09  1:51                       ` Xuan Zhuo
2023-05-09  1:51                         ` Xuan Zhuo
2023-05-07  9:31     ` David Woodhouse
2023-05-07  9:31       ` David Woodhouse
2023-05-07 13:38       ` Michael S. Tsirkin
2023-05-07 13:38         ` Michael S. Tsirkin
2023-05-07  5:59 ` Michael S. Tsirkin
2023-05-07  5:59   ` Michael S. Tsirkin

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