netdev - Re: [PATCH V2 net-next 1/4] net: ena: ethtool: use unsigned long for pointer arithmetics

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Message-ID: <pj41zlv9grp4ge.fsf@u68c7b5b1d2d758.ant.amazon.com>
Date:   Sun, 6 Sep 2020 13:47:13 +0300
From:   Shay Agroskin <shayagr@...zon.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
CC:     "Jubran, Samih" <sameehj@...zon.com>, Andrew Lunn <andrew@...n.ch>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "Machulsky, Zorik" <zorik@...zon.com>,
        "Matushevsky, Alexander" <matua@...zon.com>,
        "Bshara, Saeed" <saeedb@...zon.com>,
        "Wilson, Matt" <msw@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "Bshara, Nafea" <nafea@...zon.com>,
        "Tzalik, Guy" <gtzalik@...zon.com>,
        "Belgazal, Netanel" <netanel@...zon.com>,
        "Saidi, Ali" <alisaidi@...zon.com>,
        "Herrenschmidt, Benjamin" <benh@...zon.com>,
        "Kiyanovski, Arthur" <akiyano@...zon.com>,
        "Dagan, Noam" <ndagan@...zon.com>
Subject: Re: [PATCH V2 net-next 1/4] net: ena: ethtool: use unsigned long for pointer arithmetics


Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:

> On Thu, Aug 20, 2020 at 12:13:15PM +0000, Jubran, Samih wrote:
>> 
>> > ...
>> > 
>> > On Wed, Aug 19, 2020 at 01:43:46PM +0000, sameehj@...zon.com 
>> > wrote:
>> > > From: Sameeh Jubran <sameehj@...zon.com>
>> > >
>> > > unsigned long is the type for doing maths on pointers.
>> > 
>> > Maths on pointers is perfectly valid. The real issue here is 
>> > you have all your
>> > types mixed up.
>> 
>> The stat_offset field has the bytes from the start of the 
>> struct, the math is perfectly valid IMO¸
>> I have also went for the extra step and tested it using prints.
>> 
>> > 
>> > > -                     ptr = (u64 
>> > > *)((uintptr_t)&ring->tx_stats +
>> > > - 
>> > > (uintptr_t)ena_stats->stat_offset);
>> > > +                     ptr = (u64 *)((unsigned 
>> > > long)&ring->tx_stats +
>> > > +                             ena_stats->stat_offset);
>> > 
>> > struct ena_ring {
>> > ...
>> >         union {
>> >                 struct ena_stats_tx tx_stats;
>> >                 struct ena_stats_rx rx_stats;
>> >         };
>> > 
>> > struct ena_stats_tx {
>> >         u64 cnt;
>> >         u64 bytes;
>> >         u64 queue_stop;
>> >         u64 prepare_ctx_err;
>> >         u64 queue_wakeup;
>> >         ...
>> > }
>> > 
>> > &ring->tx_stats will give you a struct 
>> > *ena_stats_tx. Arithmetic on that,
>> > adding 1 for example, takes you forward a full ena_stats_tx 
>> > structure. Not
>> > what you want.
>> > 
>> > &ring->tx_stats.cnt however, will give you a u64 *. Adding 1 
>> > to that will give
>> > you bytes, etc.
>> 
>> 
>> If I understand you well, the alternative approach you are 
>> suggesting is:
>> 
>> ptr = &ring->tx_stats.cnt + ena_stats->stat_offset;
>
> I don't want to stir up the pot, but do you really need the 
> offsetof() of
> each member in the stats struct? Couldn't you piggyback on 
> assumption that
> these stats need to be u64 and just walk the struct with 
> pointer?
>
> 	struct ena_ring *ring;
> 	int offset;
> 	int i, j;
> 	u8 *ptr;
>
> 	for (i = 0; i < adapter->num_io_queues; i++) {
> 		/* Tx stats */
> 		ring = &adapter->tx_ring[i];
> 		ptr = (u8 *)&ring->tx_stats;
>
> 		for (j = 0; j < ENA_STATS_ARRAY_TX; j++) {
> 			ena_safe_update_stat((u64 *)ptr, 
> (*data)++, &ring->syncp);
> 			ptr += sizeof(u64);
> 		}
> 	}
>
> I find this as a simpler and lighter solution. There might be 
> issues with
> code typed in email client, but you get the idea.
>
>> 
>> of course we need to convert the stat_offset field to be in 8 
>> bytes resolution instead.
>> 
>> This approach has a potential bug hidden in it. If in the 
>> future
>> someone decides to expand the "ena_stats_tx" struct and add a 
>> field preceding cnt,
>> cnt will no longer be the beginning of the struct, which will 
>> cause a bug."
>> 
>> Therefore, if you have another way to do this, please share 
>> it. Otherwise I'd
>> rather leave this code as it is for the sake of robustness.
>> 
>> > 
>> >      Andrew

Hi all,

We tried to implement your suggestion, and found that removing the 
stat_offset
field causes problems that are challenging to solve.
Removing stat_offset introduces a requirement that the statistics 
in a stat
strings array (check [1] for example) and stat variables struct 
(check [2] for
example) must be in the same order.
This requirement is prone to future bugs that might be challenging 
to locate.
We also tried to unify the array and struct creation by
using X macros. At the moment this change requires more time and 
effort by us
and our customers need this code merged asap.

[1] https://elixir.bootlin.com/linux/v5.9-
rc3/source/drivers/net/ethernet/amazon/ena/ena_ethtool.c#L71
[2] https://elixir.bootlin.com/linux/v5.9-
rc3/source/drivers/net/ethernet/amazon/ena/ena_netdev.h#L232

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