Date:   Wed, 22 Feb 2023 16:00:30 +0100
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     brouer@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
        Stanislav Fomichev <sdf@...gle.com>, martin.lau@...nel.org,
        ast@...nel.org, daniel@...earbox.net, yoong.siang.song@...el.com,
        anthony.l.nguyen@...el.com, intel-wired-lan@...ts.osuosl.org,
        xdp-hints@...-project.net, Sasha Neftin <sasha.neftin@...el.com>
Subject: Re: [PATCH bpf-next V1] igc: enable and fix RX hash usage by netstack


On 20/02/2023 16.39, Alexander Lobakin wrote:
> From: Jesper Dangaard Brouer <jbrouer@...hat.com>
> Date: Thu, 16 Feb 2023 17:46:53 +0100
> 
>>
>> On 14/02/2023 14.21, Alexander Lobakin wrote:
>>> From: Jesper Dangaard Brouer <brouer@...hat.com>
>>> Date: Fri, 10 Feb 2023 16:07:59 +0100
>>>
>>>> When function igc_rx_hash() was introduced in v4.20 via commit
>>>> 0507ef8a0372
>>>> ("igc: Add transmit and receive fastpath and interrupt handlers"), the
>>>> hardware wasn't configured to provide RSS hash, thus it made sense to
>>>> not
>>>> enable net_device NETIF_F_RXHASH feature bit.
>>>
>>> [...]
>>>
>>>> @@ -311,6 +311,58 @@ extern char igc_driver_name[];
>>>>    #define IGC_MRQC_RSS_FIELD_IPV4_UDP    0x00400000
>>>>    #define IGC_MRQC_RSS_FIELD_IPV6_UDP    0x00800000
>>>>    +/* RX-desc Write-Back format RSS Type's */
>>>> +enum igc_rss_type_num {
>>>> +    IGC_RSS_TYPE_NO_HASH        = 0,
>>>> +    IGC_RSS_TYPE_HASH_TCP_IPV4    = 1,
>>>> +    IGC_RSS_TYPE_HASH_IPV4        = 2,
>>>> +    IGC_RSS_TYPE_HASH_TCP_IPV6    = 3,
>>>> +    IGC_RSS_TYPE_HASH_IPV6_EX    = 4,
>>>> +    IGC_RSS_TYPE_HASH_IPV6        = 5,
>>>> +    IGC_RSS_TYPE_HASH_TCP_IPV6_EX    = 6,
>>>> +    IGC_RSS_TYPE_HASH_UDP_IPV4    = 7,
>>>> +    IGC_RSS_TYPE_HASH_UDP_IPV6    = 8,
>>>> +    IGC_RSS_TYPE_HASH_UDP_IPV6_EX    = 9,
>>>> +    IGC_RSS_TYPE_MAX        = 10,
>>>> +};
>>>> +#define IGC_RSS_TYPE_MAX_TABLE        16
>>>> +#define IGC_RSS_TYPE_MASK        0xF
>>>
>>> GENMASK()?
>>>
>>
>> hmm... GENMASK(3,0) looks more confusing to me. The mask we need here is
>> so simple that I prefer not to complicate this with GENMASK.
>>

Changed my mind, I'm going with GENMASK(3,0) in V3.

>>>> +
>>>> +/* igc_rss_type - Rx descriptor RSS type field */
>>>> +static inline u8 igc_rss_type(union igc_adv_rx_desc *rx_desc)
>>>
>>> Why use types shorter than u32 on the stack?
>>
>> Changing to u32 in V2
>>
>>> Why this union is not const here, since there are no modifications?
>>
>> Sure
>>
>>>> +{
>>>> +    /* RSS Type 4-bit number: 0-9 (above 9 is reserved) */
>>>> +    return rx_desc->wb.lower.lo_dword.hs_rss.pkt_info &
>>>> IGC_RSS_TYPE_MASK;
>>>
>>> The most important I wanted to mention: doesn't this function make the
>>> CPU read the uncached field again, while you could just read it once
>>> onto the stack and then extract all such data from there?
>>
>> I really don't think this is an issues here. The igc_adv_rx_desc is only
>> 16 bytes and it should be hot in CPU cache by now.
> 
> Rx descriptors are located in the DMA coherent zone (allocated via
> dma_alloc_coherent()), I am missing something? Because I was (I am) sure
> CPU doesn't cache anything from it (and doesn't reorder reads/writes
> from/to). I thought that's the point of coherent zones -- you may talk
> to hardware without needing for syncing...
> 

That is a good point and you are (likely) right.

I do want to remind you that this is a "fixes" patch that dates back to
v5.2.  This driver is from the very beginning coded to access descriptor
this way via union igc_adv_rx_desc.  For a fixes patch, I'm not going to
code up a new and more effecient way of accessing the descriptor memory.

If you truely believe this matters for a 2.5 Gbit/s device, then someone 
(e.g you) can go through this driver and change this pattern in the code.


>>
>> To avoid the movzx I have changed this to do a u32 read instead.
>>
>>>> +}
>>>> +
>>>> +/* Packet header type identified by hardware (when BIT(11) is zero).
>>>> + * Even when UDP ports are not part of RSS hash HW still parse and
>>>> mark UDP bits
>>>> + */
>>>> +enum igc_pkt_type_bits {
>>>> +    IGC_PKT_TYPE_HDR_IPV4    =    BIT(0),
>>>> +    IGC_PKT_TYPE_HDR_IPV4_WITH_OPT=    BIT(1), /* IPv4 Hdr includes
>>>> IP options */
>>>> +    IGC_PKT_TYPE_HDR_IPV6    =    BIT(2),
>>>> +    IGC_PKT_TYPE_HDR_IPV6_WITH_EXT=    BIT(3), /* IPv6 Hdr includes
>>>> extensions */
>>>> +    IGC_PKT_TYPE_HDR_L4_TCP    =    BIT(4),
>>>> +    IGC_PKT_TYPE_HDR_L4_UDP    =    BIT(5),
>>>> +    IGC_PKT_TYPE_HDR_L4_SCTP=    BIT(6),
>>>> +    IGC_PKT_TYPE_HDR_NFS    =    BIT(7),
>>>> +    /* Above only valid when BIT(11) is zero */
>>>> +    IGC_PKT_TYPE_L2        =    BIT(11),
>>>> +    IGC_PKT_TYPE_VLAN    =    BIT(12),
>>>> +    IGC_PKT_TYPE_MASK    =    0x1FFF, /* 13-bits */
>>>
>>> Also GENMASK().
>>
>> GENMASK would make more sense here.
>>
>>>> +};
>>>> +
>>>> +/* igc_pkt_type - Rx descriptor Packet type field */
>>>> +static inline u16 igc_pkt_type(union igc_adv_rx_desc *rx_desc)
>>>
>>> Also short types and consts.
>>>
>>
>> Fixed in V2
>>
>>>> +{
>>>> +    u32 data = le32_to_cpu(rx_desc->wb.lower.lo_dword.data);
>>>> +    /* Packet type is 13-bits - as bits (16:4) in lower.lo_dword*/
>>>> +    u16 pkt_type = (data >> 4) & IGC_PKT_TYPE_MASK;
>>>
>>> Perfect candidate for FIELD_GET(). No, even for le32_get_bits().
>>

Dropping all the code and defines for "igc_pkt_type", as the code ended
up not being used.  I simply kept this around to document what was in
the programmers datasheet (to help others understand the hardware).


>> I adjusted this, but I could not find a central define for FIELD_GET
>> (but many drivers open code this).
> 
> <linux/bitfield.h>. It has FIELD_{GET,PREP}() and also builds
> {u,__le,__be}{8,16,32}_{encode,get,replace}_bits() via macro (the latter
> doesn't get indexed by Elixir, as it doesn't parse functions built via
> macros).

Thanks for the pointer to <linux/bitfield.h>, I'll be using that in V3.

>>
>>> Also my note above about excessive expensive reads.
>>>
>>>> +
>>>> +    return pkt_type;
>>>> +}
>>>> +
>>>>    /* Interrupt defines */
>>>>    #define IGC_START_ITR            648 /* ~6000 ints/sec */
>>>>    #define IGC_4K_ITR            980
>>>> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c
>>>> b/drivers/net/ethernet/intel/igc/igc_main.c
>>>> index 8b572cd2c350..42a072509d2a 100644
>>>> --- a/drivers/net/ethernet/intel/igc/igc_main.c
>>>> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
>>>> @@ -1677,14 +1677,40 @@ static void igc_rx_checksum(struct igc_ring
>>>> *ring,
>>>>               le32_to_cpu(rx_desc->wb.upper.status_error));
>>>>    }
>>>>    +/* Mapping HW RSS Type to enum pkt_hash_types */
>>>> +struct igc_rss_type {
>>>> +    u8 hash_type; /* can contain enum pkt_hash_types */
>>>
>>> Why make a struct for one field? + short type note

Also fixing this in V3.

>>>> +} igc_rss_type_table[IGC_RSS_TYPE_MAX_TABLE] = {
>>>> +    [IGC_RSS_TYPE_NO_HASH].hash_type      = PKT_HASH_TYPE_L2,
>>>> +    [IGC_RSS_TYPE_HASH_TCP_IPV4].hash_type      = PKT_HASH_TYPE_L4,
>>>> +    [IGC_RSS_TYPE_HASH_IPV4].hash_type      = PKT_HASH_TYPE_L3,
>>>> +    [IGC_RSS_TYPE_HASH_TCP_IPV6].hash_type      = PKT_HASH_TYPE_L4,
>>>> +    [IGC_RSS_TYPE_HASH_IPV6_EX].hash_type      = PKT_HASH_TYPE_L3,
>>>> +    [IGC_RSS_TYPE_HASH_IPV6].hash_type      = PKT_HASH_TYPE_L3,
>>>> +    [IGC_RSS_TYPE_HASH_TCP_IPV6_EX].hash_type = PKT_HASH_TYPE_L4,
>>>> +    [IGC_RSS_TYPE_HASH_UDP_IPV4].hash_type      = PKT_HASH_TYPE_L4,
>>>> +    [IGC_RSS_TYPE_HASH_UDP_IPV6].hash_type      = PKT_HASH_TYPE_L4,
>>>> +    [IGC_RSS_TYPE_HASH_UDP_IPV6_EX].hash_type = PKT_HASH_TYPE_L4,
>>>> +    [10].hash_type = PKT_HASH_TYPE_L2, /* RSS Type above 9
>>>> "Reserved" by HW */
>>>> +    [11].hash_type = PKT_HASH_TYPE_L2,
>>>> +    [12].hash_type = PKT_HASH_TYPE_L2,
>>>> +    [13].hash_type = PKT_HASH_TYPE_L2,
>>>> +    [14].hash_type = PKT_HASH_TYPE_L2,
>>>> +    [15].hash_type = PKT_HASH_TYPE_L2,

Changing these 10-15 to PKT_HASH_TYPE_NONE, which is zero.
The ASM generated table is smaller code size with zero padded content.

>>>
>>> Why define those empty if you could do a bound check in the code
>>> instead? E.g. `if (unlikely(bigger_than_9)) return PKT_HASH_TYPE_L2`.
>>
>> Having a branch for this is likely slower.  On godbolt I see that this
>> generates suboptimal and larger code.
> 
> But you have to verify HW output anyway, right? Or would like to rely on
> that on some weird revision it won't spit BIT(69) on you?
> 

The table is constructed such that the lookup takes care of "verifying"
the HW output.  Notice that software will bit mask the last 4 bits, thus
the number will max be 15.  No matter what hardware outputs it is safe
to do a lookup in the table.  IMHO it is a simple way to avoid an
unnecessary verification branch and still be able to handle buggy/weird
HW revs.


>>>> +};
>>>> +
>>>>    static inline void igc_rx_hash(struct igc_ring *ring,
>>>>                       union igc_adv_rx_desc *rx_desc,
>>>>                       struct sk_buff *skb)
>>>>    {
>>>> -    if (ring->netdev->features & NETIF_F_RXHASH)
>>>> -        skb_set_hash(skb,
>>>> -                 le32_to_cpu(rx_desc->wb.lower.hi_dword.rss),
>>>> -                 PKT_HASH_TYPE_L3);
>>>> +    if (ring->netdev->features & NETIF_F_RXHASH) {
>>>
>>>      if (!(feature & HASH))
>>>          return;
>>>
>>> and -1 indent level?
>>
>> Usually, yes, I also prefer early return style code.
>> For one I just followed the existing style.
> 
> I'd not recommend "keep the existing style" of Intel drivers -- not
> something I'd like to keep as is :D
> 
>>
>> Second, I tried to code it up, but it looks ugly in this case, as the
>> variable defines need to get moved outside the if statement.
>>
>>>> +        u32 rss_hash = le32_to_cpu(rx_desc->wb.lower.hi_dword.rss);
>>>> +        u8  rss_type = igc_rss_type(rx_desc);
>>>> +        enum pkt_hash_types hash_type;
>>>> +
>>>> +        hash_type = igc_rss_type_table[rss_type].hash_type;
>>>> +        skb_set_hash(skb, rss_hash, hash_type);
>>>> +    }
>>>>    }
>>>
>>> [...]

--Jesper

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