Re: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU
On Tue, Dec 4, 2018 at 9:06 AM Eric Dumazet wrote: > > > > On 12/04/2018 08:59 AM, David Laight wrote: > > Where does 68 come from? > > Min IPv4 MTU per RFC791 > What's wrong with using IPV4_MIN_MTU instead of 68 even in just comment?
RE: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU
From: Eric Dumazet > Sent: 04 December 2018 17:04 > On 12/04/2018 08:59 AM, David Laight wrote: > > From: Tariq Toukan > >> Sent: 02 December 2018 12:35 > >> From: Eran Ben Elisha > >> > >> NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in > >> the RFC791 and in the network stack. Remove old mlx4_en only define for > >> it, which was set to wrong value. > > ... > >> > >> - /* MTU range: 46 - hw-specific max */ > >> - dev->min_mtu = MLX4_EN_MIN_MTU; > >> + /* MTU range: 68 - hw-specific max */ > >> + dev->min_mtu = ETH_MIN_MTU; > >>dev->max_mtu = priv->max_mtu; > > > > Where does 68 come from? > > Min IPv4 MTU per RFC791 Maybe I'm just confused and these are the ranges that the 'maximum mtu' can be set to. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
RE: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU
From: Eric Dumazet > Sent: 04 December 2018 17:04 > > On 12/04/2018 08:59 AM, David Laight wrote: > > From: Tariq Toukan > >> Sent: 02 December 2018 12:35 > >> From: Eran Ben Elisha > >> > >> NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in > >> the RFC791 and in the network stack. Remove old mlx4_en only define for > >> it, which was set to wrong value. > > ... > >> > >> - /* MTU range: 46 - hw-specific max */ > >> - dev->min_mtu = MLX4_EN_MIN_MTU; > >> + /* MTU range: 68 - hw-specific max */ > >> + dev->min_mtu = ETH_MIN_MTU; > >>dev->max_mtu = priv->max_mtu; > > > > Where does 68 come from? > > Min IPv4 MTU per RFC791 Which has nothing to do with an ethernet driver. Indeed, IIRC, it is the smallest maximum frame size that IPv4 can work over. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Re: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU
On 12/04/2018 08:59 AM, David Laight wrote: > From: Tariq Toukan >> Sent: 02 December 2018 12:35 >> From: Eran Ben Elisha >> >> NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in >> the RFC791 and in the network stack. Remove old mlx4_en only define for >> it, which was set to wrong value. > ... >> >> -/* MTU range: 46 - hw-specific max */ >> -dev->min_mtu = MLX4_EN_MIN_MTU; >> +/* MTU range: 68 - hw-specific max */ >> +dev->min_mtu = ETH_MIN_MTU; >> dev->max_mtu = priv->max_mtu; > > Where does 68 come from? Min IPv4 MTU per RFC791 > The minimum size of an ethernet packet including the mac addresses > and CRC is 64 bytes - but that would never be an 'mtu'. > > Since 64 - 46 = 18, the 46 probably excludes both MAC addresses, > the ethertype/length and the CRC. > This is 'sort of' the minimum mtu for an ethernet frame. > > I'm not sure which values are supposed to be in dev->min/max_mtu. I am not sure we really care, only syzkaller can possibly.
RE: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU
From: Tariq Toukan > Sent: 02 December 2018 12:35 > From: Eran Ben Elisha > > NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in > the RFC791 and in the network stack. Remove old mlx4_en only define for > it, which was set to wrong value. ... > > - /* MTU range: 46 - hw-specific max */ > - dev->min_mtu = MLX4_EN_MIN_MTU; > + /* MTU range: 68 - hw-specific max */ > + dev->min_mtu = ETH_MIN_MTU; > dev->max_mtu = priv->max_mtu; Where does 68 come from? The minimum size of an ethernet packet including the mac addresses and CRC is 64 bytes - but that would never be an 'mtu'. Since 64 - 46 = 18, the 46 probably excludes both MAC addresses, the ethertype/length and the CRC. This is 'sort of' the minimum mtu for an ethernet frame. I'm not sure which values are supposed to be in dev->min/max_mtu. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
