Re: [MirageOS-devel] wireshark capture of failed download from mirage-www on ARM

[Thomas Leonard <talex5@xxxxxxxxx>]

On 22 July 2014 04:00, Anil Madhavapeddy <anil@xxxxxxxxxx> wrote:
> On 21 Jul 2014, at 16:10, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>
>> What are the rules about alignment for cstructs?
>>
>> Allocating the buffer like this works:
>>
>>  let data = Io_page.get 1 |> Io_page.to_cstruct
>>  let () = Cstruct.blit_from_string (Buffer.contents buffer) 0 data 0
>> (Buffer.length buffer)
>>
>> But using Cstruct.of_string doesn't. It does look like Netif assumes
>> the structs are page aligned. e.g.
>>
>>  if page.Cstruct.off + len > page_size then begin
>>    (* netback rejects packets that cross page boundaries *)
>>    let msg =
>>      Printf.sprintf "Invalid page: offset=%d, length=%d"
>> page.Cstruct.off len in
>>    print_endline msg;
>>    Lwt.fail (Failure msg)
>>  end else
>
> Argh, this is exactly the problem.  Netif requires page aligned buffers
> (in theory, sub-page grants are possible, but ill-advised for performance
> reasons).
>
> Cstruct.of_string calls Cstruct.create, which calls Bigarray.Array1.create
> which isn't page-aligned.  It does need to go through Io_page to ensure it's
> page-aligned.
>
> We don't protect this distinction using phantom types, and it's bitten us
> several times now through these hard-to-spot dynamic failures :-/
>
> Making Cstruct's always page aligned is too expensive for the 'casual'
> small Cstructs (as seen in OCaml TLS for example), so ensure your buffer
> originates from Io_page is the best bet for now.  An issue on how to
> ensure this is checked statically would be good to have (but involve
> a fair bit of mechanical code motion).

OK, that helps.

On ARM, it still sometimes needs to retransmit though. In some
packets, only the first 128 bytes are correct and the rest are zero
(according to wireshark). The buffer I'm sending is statically
allocated, so it can't be getting freed too soon. How can the net
driver see zeros in the same physical page in which it already saw the
first part of the data?

My test repository is here:

  https://github.com/talex5/net-problem


-- 
Dr Thomas Leonard        http://0install.net/
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