netdev - RE: [PATCH v4 2/2] bnx2x: disable GSO where gso

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Date:   Thu, 01 Feb 2018 08:11:27 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     "Chopra\, Manish" <Manish.Chopra@...ium.com>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Jason Wang <jasowang@...hat.com>,
        Pravin Shelar <pshelar@....org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: RE: [PATCH v4 2/2] bnx2x: disable GSO where gso_size is too big for hardware

"Chopra, Manish" <Manish.Chopra@...ium.com> writes:

>> -----Original Message-----
>> From: Daniel Axtens [mailto:dja@...ens.net]
>> Sent: Wednesday, January 31, 2018 8:46 AM
>> To: netdev@...r.kernel.org
>> Cc: Daniel Axtens <dja@...ens.net>; Eric Dumazet <eric.dumazet@...il.com>;
>> Chopra, Manish <Manish.Chopra@...ium.com>; Jason Wang
>> <jasowang@...hat.com>; Pravin Shelar <pshelar@....org>; Marcelo Ricardo
>> Leitner <marcelo.leitner@...il.com>
>> Subject: [PATCH v4 2/2] bnx2x: disable GSO where gso_size is too big for
>> hardware
>> 
>> If a bnx2x card is passed a GSO packet with a gso_size larger than
>> ~9700 bytes, it will cause a firmware error that will bring the card
>> down:
>> 
>> bnx2x: [bnx2x_attn_int_deasserted3:4323(enP24p1s0f0)]MC assert!
>> bnx2x: [bnx2x_mc_assert:720(enP24p1s0f0)]XSTORM_ASSERT_LIST_INDEX 0x2
>> bnx2x: [bnx2x_mc_assert:736(enP24p1s0f0)]XSTORM_ASSERT_INDEX 0x0 =
>> 0x00000000 0x25e43e47 0x00463e01 0x00010052
>> bnx2x: [bnx2x_mc_assert:750(enP24p1s0f0)]Chip Revision: everest3, FW
>> Version: 7_13_1 ... (dump of values continues) ...
>> 
>> Detect when the mac length of a GSO packet is greater than the maximum
>> packet size (9700 bytes) and disable GSO.
>> 
>> Signed-off-by: Daniel Axtens <dja@...ens.net>
>> 
>> ---
>> 
>> v4: Only call the slow check if the gso_size is large.
>>     Eric - I think this is what you had in mind?
>>     Manish - do you think this is an acceptable performance trade-off?
>>              GSO will work for any packet size, and only jumbo frames will
>> 	     have to do the slower test.
>> 
>> Again, only build-tested.
>> ---
>>  drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c | 18
>> ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>> 
>> diff --git a/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
>> b/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
>> index 7b08323e3f3d..74fc9af4aadb 100644
>> --- a/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
>> +++ b/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
>> @@ -12934,6 +12934,24 @@ static netdev_features_t
>> bnx2x_features_check(struct sk_buff *skb,
>>  					      struct net_device *dev,
>>  					      netdev_features_t features)
>>  {
>> +	/*
>> +	 * A skb with gso_size + header length > 9700 will cause a
>> +	 * firmware panic. Drop GSO support.
>> +	 *
>> +	 * Eventually the upper layer should not pass these packets down.
>> +	 *
>> +	 * For speed, if the gso_size is <= 9000, assume there will
>> +	 * not be 700 bytes of headers and pass it through. Only do a
>> +	 * full (slow) validation if the gso_size is > 9000.
>> +	 *
>> +	 * (Due to the way SKB_BY_FRAGS works this will also do a full
>> +	 * validation in that case.)
>> +	 */
>> +	if (unlikely(skb_is_gso(skb) &&
>> +		     (skb_shinfo(skb)->gso_size > 9000) &&
>> +		     !skb_gso_validate_mac_len(skb, 9700)))
>> +		features &= ~NETIF_F_GSO_MASK;
>
> Hi Daniel,
>
> Obviously, it could be bad from performance perspective since every gso packet has to do these check.
> When running iperf/netperf performance benchmark, where GSO is likely to occur.
>
> Why do you have to put two checks for skb_is_gso() and gso_size ? Isn't gso_size > anything means GSO skb ?

Yes, the check is redundant. I do it to make my logic clearer.

The compiler will optimise it into one check. I compiled with gcc-7.2
and gcc-4.8, both targeting amd64, and in both cases I could only find
one relevant cmp:

skb_is_gso():
/home/dja/dev/linux/linux/include/linux/skbuff.h:4032
    3686:       48 8b 8f d0 00 00 00    mov    0xd0(%rdi),%rcx
bnx2x_features_check():
/home/dja/dev/linux/linux/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c:12950
    368d:       66 81 7c 01 04 28 23    cmpw   $0x2328,0x4(%rcx,%rax,1)
    3694:       0f 87 ea 01 00 00       ja     3884 <bnx2x_features_check+0x214>

0x2328 is decimal 9000.

> I assume it won't cause disabling the offload if "headers [L2 + L3 + L4] + gso_size" is still <= 9700. ?

That is correct. The flow is:

If the gso_size is less than or equal to 9000, offload is enabled with
no further checks. This is the fast path.

If the gso_size is greater than 9000, then the "headers [L2 + L3 + L4] +
gso_size" is checked. This is an out-of-line check done once per GSO skb.

If headers + gso_size is <= 9700, offload is enabled.

If headers + gso_size > 9700, offload is disabled.

Regards,
Daniel
>
> Thanks,
> Manish