[dpdk-dev] [PATCH 1/2] net/tap: add tun support

Varghese, Vipin vipin.varghese at intel.com
Thu May 3 07:59:05 CEST 2018
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Hi Ophir,

Shall I investigate and implement the suggestion from Ferruh to use variable logic?

Thanks
Vipin Varghese

> -----Original Message-----
> From: Yigit, Ferruh
> Sent: Monday, April 23, 2018 9:12 PM
> To: Varghese, Vipin <vipin.varghese at intel.com>; Ophir Munk
> <ophirmu at mellanox.com>; dev at dpdk.org; pascal.mazon at 6wind.com;
> Thomas Monjalon <thomas at monjalon.net>; Olga Shern
> <olgas at mellanox.com>; Shahaf Shuler <shahafs at mellanox.com>
> Subject: Re: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
> 
> On 4/23/2018 1:58 PM, Varghese, Vipin wrote:
> > Hi Ophir,
> >
> > Can you help me with the investigation with the following information?
> > 1) The kernel or distro in which the TAP proto flag set breaks the logic?
> 
> Hi Vipin,
> 
> I guess Ophir's point is not this is broken with some kernels but a valid field
> set wrong for tap, perhaps someone can be using a custom kernel module to
> use those fields, we can't know it.
> 
> Instead of duplicating [rt]x_burst() functions, I suggest creating a variable to
> set if this is tun or tap and set pi.proto only for tun, this will lead less
> comparison for tap and correct proto value.
> 
> > 2) Is the above still valid even after applying the patch '
> https://dpdk.org/dev/patchwork/patch/37986/'?
> 
> I guess his concern is for tap, that some MAC addresses cause wrong
> pi.proto, not for tun which your patch fixes.
> 
> >
> > Note: I am testing with 3.13.0, 4.4.0 and 4.13.0.
> >
> > Thanks
> > Vipin Varghese
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Varghese, Vipin
> >> Sent: Saturday, April 21, 2018 8:40 PM
> >> To: Ophir Munk <ophirmu at mellanox.com>; dev at dpdk.org;
> >> pascal.mazon at 6wind.com; Yigit, Ferruh <ferruh.yigit at intel.com>;
> >> Thomas Monjalon <thomas at monjalon.net>; Olga Shern
> >> <olgas at mellanox.com>; Shahaf Shuler <shahafs at mellanox.com>
> >> Subject: Re: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
> >>
> >> Hi Ophir,
> >>
> >> <Snip>
> >>
> >>> Hi Vipin,
> >>> I missed your point:
> >>> You claim that TAP should work regardless of any pi.proto values.
> >>> Can you confirm that for ALL kernels versions (past and future)?
> >>
> >> I have tested with 3.13.0 , 4.4.0 with patch fix.
> >>
> >>>
> >>>> -----Original Message-----
> >>>> From: Ophir Munk
> >>>> Sent: Saturday, April 21, 2018 12:49 AM
> >>>> To: Varghese, Vipin <vipin.varghese at intel.com>; dev at dpdk.org;
> >>>> pascal.mazon at 6wind.com; Yigit, Ferruh <ferruh.yigit at intel.com>;
> >>>> Thomas Monjalon <thomas at monjalon.net>; Olga Shern
> >>>> <olgas at mellanox.com>; Shahaf Shuler <shahafs at mellanox.com>
> >>>> Subject: RE: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
> >>>>
> >>>> Hi Vipin,
> >>>>
> >>>> Please find comments inline.
> >>>>
> >>>>> -----Original Message-----
> >>>>> From: Varghese, Vipin [mailto:vipin.varghese at intel.com]
> >>>>> Sent: Friday, April 13, 2018 6:18 AM
> >>>>> To: Ophir Munk <ophirmu at mellanox.com>; dev at dpdk.org;
> >>>>> pascal.mazon at 6wind.com; Yigit, Ferruh <ferruh.yigit at intel.com>;
> >>>>> Thomas Monjalon <thomas at monjalon.net>; Olga Shern
> >>>>> <olgas at mellanox.com>; Shahaf Shuler <shahafs at mellanox.com>
> >>>>> Subject: RE: [dpdk-dev] [PATCH 1/2] net/tap: add tun support
> >>>>>
> >>
> >> <Snip>
> >>
> >>>>>> 1. Accessing the first byte here assumes it is the first IP
> >>>>>> header byte (layer 3) which is correct for TUN.
> >>>>>> For TAP however the first byte belongs to Ethernet destination
> >>>>>> address (layer 2).
> >>>>>> Please explain how this logic will work for TAP.
> >>>>>
> >>>>> Based on linux code base '/driver/net/tap.c' and '/driver/net/tun.c'
> >>>>> from 3.13. to  4.16,
> >>>>>
> >>>>> Please find my observation below
> >>>>> 1. File: tun.c, function: tun_get_user, check for 'tun->flags &
> >>>>> TUN_TYPE_MASK' is done and if non ip is taken counter 'rx_dropped'
> >>>>> is updated.
> >>>>> 2. File: tap.c, there are no checks for 'tap->flags' for IFF_NO_PI
> >>>>> in rx data path. Counter 'rx_dropped' is updated in
> 'tap_handle_frame'.
> >>>>>
> >>>>
> >>>> I understand that in kernel implementation there is no check for
> >>>> tap->flags in file tap.c, however I think there is a bug in dpdk
> >>>> tap->rte_eth_tap.c
> >>> file.
> >>>> Please find below an example which demonstrates this claim.
> >>>>
> >>>>> Please find my reasoning below
> >>>>> 1. First approach was to have separate function for tap and tun TX
> >>>>> and
> >> RX.
> >>>>> But this will introduce code duplication, hence reworked the code
> >>>>> as
> >>>> above.
> >>>>
> >>>> I agree. Avoiding code duplication is a good approach.
> >>>>
> >>>>> 2. During my internal testing assigning dummy value for protocol
> >>>>> field in TAP packets, did not show a difference in behaviour. May
> >>>>> be there are some specific cases this failing.
> >>>>>
> >>>>> If there difference in behaviour, can please share the same?
> >>>>>
> >>>>
> >>>> Please consider the following example:
> >>>> I am running testpmd with a TAP device, --forward-mode=csum.
> >>>> I am injecting a TCP packet, which is forwarded back (mac addresses
> >>>> swapped) to the sender.
> >>>> Using gdb I set a breakpoint at pmd_tx_burst() in file
> >>>> rte_eth_tap.c
> >>>>
> >>>> Looking at the following code inside pmd_tx_burst():
> >>>>
> >>>> 527                 char *buff_data = rte_pktmbuf_mtod(seg, void *);
> >>>> 528                 j = (*buff_data & 0xf0);
> >>>> 529                 pi.proto = (j == 0x40) ? 0x0008 :
> >>>> 530                                 (j == 0x60) ? 0xdd86 : 0x00;
> >>>>
> >>>> I am printing the first 20 bytes of buff_data in line 527:
> >>>>
> >>>> (gdb) p/x *(unsigned char *)buff_data at 20
> >>>> $3 = {0x0, 0x25, 0x88, 0x10, 0x66, 0x2, 0xf4, 0x52, 0x14, 0x7a,
> >>>> 0x59, 0x81, 0x8, 0x0, 0x45, 0x0, 0x4, 0xdf, 0x0, 0x1}
> >>>>
> >>>> The gdb printout refers to:
> >>>> 6 bytes of destination MAC address: 0x0, 0x25, 0x88, 0x10, 0x66,
> >>>> 0x2
> >>>> 6 bytes of source MAC address: 0xf4, 0x52, 0x14, 0x7a, 0x59, 0x81
> >>>> 2 bytes of Ethernet type: 0x8, 0x0 - (IPv4) IP header starting with 0x45,
> ...
> >>>> which is the byte (0x45) that "j" should have looked at
> >>>>
> >>>> In the case of TAP - buff_data starts with the destination MAC
> >>>> address of the sender (0x0, ...).
> >>>> The code in line 528 expects that buff_data would start with an IP
> >>>> header protocol (e.g. 0x45), but it is not the case for TAP.
> >>>> In my case j=0x0 (line 528) which is harmless (as it ends up with
> >>>> setting pi.proto=0x00, which is correct for TAP).
> >>>> However, if the sender had an Intel NIC - the destination MAC
> >>>> address could have started with:
> >>>> $3 = {0x40, 0x25, 0xC2, ...
> >>>> Or-
> >>>> $3 = {0x64, 0xD4, 0xDA, ...
> >>>>
> >>>> as 4025C2 and 64D4DA are reserved prefixes for Intel Ethernet MAC
> >>>> addresses, see: http://www.coffer.com/mac_find/?string=intel
> >>>>
> >>>> In this case pi.proto could end up with 0x0008 or 0xdd86 instead of
> >>>> 0x0 as expected for TAP.
> >>>>
> >>>> I hope that this example clarifies the bug I am referring to.
> >>>>
> >>
> >> Thanks for sharing detailed example overview. But as you mentioned
> >> this will break ' 4025C2' and ' 64D4DA', This will not solve for the correction
> patch  '
> >> https://dpdk.org/dev/patchwork/patch/37986/'.
> >>
> >> Only choice left is separate tx_burst for TAP and TUN PMD, as we do
> >> not want to check PMD type on each call.
> >>
> >> Questions:
> >> 1) Is this ok to split tx_burst and have redundant code?
> >> 2) Does applications transparently send packets coming from Physical
> >> NIC to TAP interface? Does not the application Modifies the DEST MAC
> >> addr to TAP interface?
> >>
> >>>>>>
> >>>>>> 2. If the first TUN byte contains 0x2X (which is neither IPv4 nor
> >>>>>> IPv6) it will end up by setting ip.proto as 0xdd86.
> >>>>>> Please explain how this logic will work for non-IP packets in TUN
> >>>>>
> >>>>> I see your point. You are correct about this. Thanks for pointing
> >>>>> out, may I send correction for this as
> >>>>>
> >>>>> """
> >>>>> -		if (j & (0x40 | 0x60))
> >>>>> -			pi.proto = (j == 0x40) ? 0x0008 : 0xdd86;
> >>>>> +		pi.proto = (j == 0x40) ? 0x0008 :
> >>>>> +					(j == 0x60) ? 0xdd86 :
> >>>>> +					0x00;
> >>>>> """
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