linux-kernel - Re: [PATCH] [v2] infiniband: i40iw, nes: don't use wall time for TCP sequence numbers

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Date:   Fri, 29 Jun 2018 17:10:52 -0500
From:   Shiraz Saleem <shiraz.saleem@...el.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Latif, Faisal" <faisal.latif@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "David S. Miller" <davem@...emloft.net>,
        "Orosco, Henry" <henry.orosco@...el.com>,
        "Nikolova, Tatyana E" <tatyana.e.nikolova@...el.com>,
        "Ismail, Mustafa" <mustafa.ismail@...el.com>,
        Bart Van Assche <bart.vanassche@....com>,
        Yuval Shaia <yuval.shaia@...cle.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] [v2] infiniband: i40iw, nes: don't use wall time for TCP
 sequence numbers

On Wed, Jun 27, 2018 at 07:26:05AM -0600, Arnd Bergmann wrote:
> The nes infiniband driver uses current_kernel_time() to get a nanosecond
> granunarity timestamp to initialize its tcp sequence counters. This is
> one of only a few remaining users of that deprecated function, so we
> should try to get rid of it.
> 
> Aside from using a deprecated API, there are several problems I see here:
> 
> - Using a CLOCK_REALTIME based time source makes it predictable in
>   case the time base is synchronized.
> - Using a coarse timestamp means it only gets updated once per jiffie,
>   making it even more predictable in order to avoid having to access
>   the hardware clock source
> - The upper 2 bits are always zero because the nanoseconds are at most
>   999999999.
> 
> For the Linux TCP implementation, we use secure_tcp_seq(), which appears
> to be appropriate here as well, and solves all the above problems.
> 
> i40iw uses a variant of the same code, so I do that same thing there
> for ipv4. Unlike nes, i40e also supports ipv6, which needs to call
> secure_tcpv6_seq instead.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> v2: use secure_tcpv6_seq for IPv6 support as suggested by Shiraz Saleem.
> ---

Looks good.

Acked-by: Shiraz Saleem <shiraz.saleem@...el.com>