Re: [PATCH bpf-next 0/4] i40e AF_XDP zero-copy buffer leak fixes

["Björn Töpel" <bjorn.topel@gmail.com>]

Den ons 5 sep. 2018 kl 19:14 skrev Jakub Kicinski
<jakub.kicinski@netronome.com>:
>
> On Tue,  4 Sep 2018 20:11:01 +0200, Björn Töpel wrote:
> > From: Björn Töpel <bjorn.topel@intel.com>
> >
> > This series addresses an AF_XDP zero-copy issue that buffers passed
> > from userspace to the kernel was leaked when the hardware descriptor
> > ring was torn down.
> >
> > The patches fixes the i40e AF_XDP zero-copy implementation.
> >
> > Thanks to Jakub Kicinski for pointing this out!
> >
> > Some background for folks that don't know the details: A zero-copy
> > capable driver picks buffers off the fill ring and places them on the
> > hardware Rx ring to be completed at a later point when DMA is
> > complete. Similar on the Tx side; The driver picks buffers off the Tx
> > ring and places them on the Tx hardware ring.
> >
> > In the typical flow, the Rx buffer will be placed onto an Rx ring
> > (completed to the user), and the Tx buffer will be placed on the
> > completion ring to notify the user that the transfer is done.
> >
> > However, if the driver needs to tear down the hardware rings for some
> > reason (interface goes down, reconfiguration and such), the userspace
> > buffers cannot be leaked. They have to be reused or completed back to
> > userspace.
> >
> > The implementation does the following:
> >
> > * Outstanding Tx descriptors will be passed to the completion
> >   ring. The Tx code has back-pressure mechanism in place, so that
> >   enough empty space in the completion ring is guaranteed.
> >
> > * Outstanding Rx descriptors are temporarily stored on a stash/reuse
> >   queue. The reuse queue is based on Jakub's RFC. When/if the HW rings
> >   comes up again, entries from the stash are used to re-populate the
> >   ring.
> >
> > * When AF_XDP ZC is enabled, disallow changing the number of hardware
> >   descriptors via ethtool. Otherwise, the size of the stash/reuse
> >   queue can grow unbounded.
> >
> > Going forward, introducing a "zero-copy allocator" analogous to Jesper
> > Brouer's page pool would be a more robust and reuseable solution.
> >
> > Jakub: I've made a minor checkpatch-fix to your RFC, prior adding it
> > into this series.
>
> Thanks for the fix! :)
>
> Out of curiosity, did checking the reuse queue have a noticeable impact
> in your test (i.e. always using the _rq() helpers)?  You seem to be
> adding an indirect call, would that not be way worse on a retpoline
> kernel?

Do you mean the indirection in __i40e_alloc_rx_buffers_zc (patch #3)?
The indirect call is elided by the __always_inline -- without that
retpoline took 2.5Mpps worth of Rx. :-(

I'm only using the _rq helpers in the configuration/slow path, so the
fast-path is unchanged.


Björn