linux-kernel - Re: [PATCH] NTB: ntb_perf: fix cast to restricted _

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Message-ID: <CAK8P3a2AUWfhg+srzt9m=oj4_jN4BDg3bWrwK3dDN1tNsGoqMg@mail.gmail.com>
Date:   Fri, 19 Jan 2018 22:26:37 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     jdmason@...zu.us, Dave Jiang <dave.jiang@...el.com>,
        Allen.Hubbe@...il.com, "Hook, Gary" <gary.hook@....com>,
        Sergey.Semin@...latforms.ru, linux-ntb@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] NTB: ntb_perf: fix cast to restricted __le32

On Fri, Jan 19, 2018 at 10:03 PM, Serge Semin <fancer.lancer@...il.com> wrote:
>
> Actually the provided patch is the best solution I could come up with.
> The thing is, that the methods can't be changed. Those functions are
> the part of the NTB API methods used by many drivers. So basically they
> are like pci_{read,write}_config_{byte,word,dword}() methods. We can't
> change their prototypes only because it's suit some driver. The methods
> give an access to the NTB device dummy u32-sized registers, nothing
> else. So endianness is the transmitted data settings in this case.
>
> NTB is the technology to interconnect some two systems with possibly
> different endianness (unlike PCI, which interconnect CPU with LE devices).
> In this case I'd need to set some agreement up between two systems about
> the endianness of the exchanged data like host and network types in
> Linux networking. I've chosen the network data to be little-endian,
> that's why I needed first to convert them from CPU to le32, then on
> remote side convert them back from le32 to CPU.
>
> If you have any better suggestion how the warning can be fixed, I'd
> be glad to stick to it.

I don't think your description matches what you actually do: The
underlying ntb hardware drivers (amd, idt, intel, mscc) all treat the
incoming data as CPU-endian and convert it to little-endian on
the register side, so the framework already assumes that whatever
you do here uses a little-endian wire-level protocol.

On a little-endian kernel/CPU, nothing is ever swapped here, neither
in the ntb_perf front-end nor in the back-ends. On a big-endian
kernel/CPU, they both swap, so you end up with CPU-endian
data on the wire, so it should be impossible for a big-endian
system to talk to a little-endian one. Have you actually tried that
combination with the current code?

If my interpretation is correct, then the best solution would be to
completely remove the cpu_to_le32/le32_to_cpu conversions
from ntb_perf, and just define that it works like any other PCI
device, exchanging little-endian data.

There are two interesting cases to consider though:

- if someone wants to implement an NTB based protocol
  using big-endian data on the wire, you probably want to add
  a ntb_peer_spad_read_be()/ntb_peer_msg_write_be()
  set of interfaces, to go along with ioread32_be()/iowrite32_be()
  the same way that ntb_peer_spad_read()/ntb_peer_msg_write()
  ends up doing ioread32()/iowrite32() with the implied little-endian
  behavior.

- memcpy_toio()/memcpy_fromio() and ioread32_rep()/iowrite32_rep
  importantly do not do any byteswap, they are meant to
  transfer byte streams.

        Arnd