netdev - Re: [PATCH 00/11] XDP unaligned chunk placement support

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Date:   Fri, 28 Jun 2019 13:29:02 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Laatz, Kevin" <kevin.laatz@...el.com>
Cc:     "Jakub Kicinski" <jakub.kicinski@...ronome.com>,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        bjorn.topel@...el.com, magnus.karlsson@...el.com,
        bpf@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        bruce.richardson@...el.com, ciara.loftus@...el.com
Subject: Re: [PATCH 00/11] XDP unaligned chunk placement support



On 28 Jun 2019, at 9:19, Laatz, Kevin wrote:

> On 27/06/2019 22:25, Jakub Kicinski wrote:
>> On Thu, 27 Jun 2019 12:14:50 +0100, Laatz, Kevin wrote:
>>> On the application side (xdpsock), we don't have to worry about the 
>>> user
>>> defined headroom, since it is 0, so we only need to account for the
>>> XDP_PACKET_HEADROOM when computing the original address (in the 
>>> default
>>> scenario).
>> That assumes specific layout for the data inside the buffer.  Some 
>> NICs
>> will prepend information like timestamp to the packet, meaning the
>> packet would start at offset XDP_PACKET_HEADROOM + metadata len..
>
> Yes, if NICs prepend extra data to the packet that would be a problem 
> for
> using this feature in isolation. However, if we also add in support 
> for in-order
> RX and TX rings, that would no longer be an issue. However, even for 
> NICs
> which do prepend data, this patchset should not break anything that is 
> currently
> working.

I read this as "the correct buffer address is recovered from the shadow 
ring".
I'm not sure I'm comfortable with that, and I'm also not sold on 
in-order completion
for the RX/TX rings.



>> I think that's very limiting.  What is the challenge in providing
>> aligned addresses, exactly?
> The challenges are two-fold:
> 1) it prevents using arbitrary buffer sizes, which will be an issue 
> supporting e.g. jumbo frames in future.
> 2) higher level user-space frameworks which may want to use AF_XDP, 
> such as DPDK, do not currently support having buffers with 'fixed' 
> alignment.
>     The reason that DPDK uses arbitrary placement is that:
>         - it would stop things working on certain NICs which 
> need the actual writable space specified in units of 1k - therefore we 
> need 2k + metadata space.
>         - we place padding between buffers to avoid constantly 
> hitting the same memory channels when accessing memory.
>         - it allows the application to choose the actual buffer 
> size it wants to use.
>     We make use of the above to allow us to speed up processing 
> significantly and also reduce the packet buffer memory size.
>
>     Not having arbitrary buffer alignment also means an AF_XDP 
> driver for DPDK cannot be a drop-in replacement for existing drivers 
> in those frameworks. Even with a new capability to allow an arbitrary 
> buffer alignment, existing apps will need to be modified to use that 
> new capability.

Since all buffers in the umem are the same chunk size, the original 
buffer
address can be recalculated with some multiply/shift math.  However, 
this is
more expensive than just a mask operation.
-- 
Jonathan