Re: [PATCH RFC v1 05/15] ixgbe: add XDP frame size to driver

Alexander Duyck
2020-03-20T22:37:46Z

On Fri, Mar 20, 2020 at 2:44 PM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:
>
> On Wed, 18 Mar 2020 14:23:09 -0700
> Alexander Duyck <alexander.duyck@gmail.com> wrote:
>
> > On Wed, Mar 18, 2020 at 1:04 PM Maciej Fijalkowski
> > <maciej.fijalkowski@intel.com> wrote:
> > >
> > > On Tue, Mar 17, 2020 at 06:29:33PM +0100, Jesper Dangaard Brouer wrote:
> > > > The ixgbe driver uses different memory models depending on PAGE_SIZE at
> > > > compile time. For PAGE_SIZE 4K it uses page splitting, meaning for
> > > > normal MTU frame size is 2048 bytes (and headroom 192 bytes).
> > >
> > > To be clear the 2048 is the size of buffer given to HW and we slice it up
> > > in a following way:
> > > - 192 bytes dedicated for headroom
> > > - 1500 is max allowed MTU for this setup
> > > - 320 bytes for tailroom (skb shinfo)
> > >
> > > In case you go with higher MTU then 3K buffer would be used and it would
> > > came from order1 page and we still do the half split. Just FYI all of this
> > > is for PAGE_SIZE == 4k and L1$ size == 64.
> >
> > True, but for most people this is the most common case since these are
> > the standard for x86.
> >
> > > > For PAGE_SIZE larger than 4K, driver advance its rx_buffer->page_offset
> > > > with the frame size "truesize".
> > >
> > > Alex, couldn't we base the truesize here somehow on ixgbe_rx_bufsz() since
> > > these are the sizes that we are passing to hw? I must admit I haven't been
> > > in touch with systems with PAGE_SIZE > 4K.
> >
> > With a page size greater than 4K we can actually get many more uses
> > out of a page by using the frame size to determine the truesize of the
> > packet. The truesize is the memory footprint currently being held by
> > the packet. So once the packet is filled we just have to add the
> > headroom and tailroom to whatever the hardware wrote instead of having
> > to use what we gave to the hardware. That gives us better efficiency,
> > if we used ixgbe_rx_bufsz() we would penalize small packets and that
> > in turn would likely hurt performance.
> >
> > > >
> > > > When driver enable XDP it uses build_skb() which provides the necessary
> > > > tailroom for XDP-redirect.
> > >
> > > We still allow to load XDP prog when ring is not using build_skb(). I have
> > > a feeling that we should drop this case now.
> > >
> > > Alex/John/Bjorn WDYT?
> >
> > The comment Jesper had about using using build_skb() when XDP is in
> > use is incorrect. The two are not correlated. The underlying buffer is
> > the same, however we drop the headroom and tailroom if we are in
> > _RX_LEGACY mode. We default to build_skb and the option of switching
> > to legacy Rx is controlled via the device private flags.
>
> Thanks for catching that.
>
> > However with that said the change itself is mostly harmless, and
> > likely helps to resolve issues that would be seen if somebody were to
> > enable XDP while having the RX_LEGACY flag set.
>
> So what is the path forward(?). Are you/Intel okay with disallowing
> XDP when the RX_LEGACY flag is set?

Why would we need to disallow it? It won't work for the redirect use
case, but other use cases should work just fine. I thought with this
patch set you were correctly reporting the headroom or tailroom so
that we would either reallocate or just drop the frame if it cannot be
handled.