Re: [PATCH net v4 1/3] ixgbe: allow to increase MTU to 3K with XDP enabled

[Alexander H Duyck <alexander.duyck@gmail.com>]

On Wed, 2023-02-08 at 17:27 +0100, Maciej Fijalkowski wrote:
> On Wed, Feb 08, 2023 at 07:37:57AM -0800, Alexander H Duyck wrote:
> > On Wed, 2023-02-08 at 10:43 +0800, Jason Xing wrote:
> > > From: Jason Xing <kernelxing@tencent.com>
> > > 
> > > Recently I encountered one case where I cannot increase the MTU size
> > > directly from 1500 to a much bigger value with XDP enabled if the
> > > server is equipped with IXGBE card, which happened on thousands of
> > > servers in production environment. After appling the current patch,
> > > we can set the maximum MTU size to 3K.
> > > 
> > > This patch follows the behavior of changing MTU as i40e/ice does.
> > > 
> > > Referrences:
> > > [1] commit 23b44513c3e6 ("ice: allow 3k MTU for XDP")
> > > [2] commit 0c8493d90b6b ("i40e: add XDP support for pass and drop actions")
> > > 
> > > Fixes: fabf1bce103a ("ixgbe: Prevent unsupported configurations with XDP")
> > > Signed-off-by: Jason Xing <kernelxing@tencent.com>
> > 
> > This is based on the broken premise that w/ XDP we are using a 4K page.
> > The ixgbe driver isn't using page pool and is therefore running on
> > different limitations. The ixgbe driver is only using 2K slices of the
> > 4K page. In addition that is reduced to 1.5K to allow for headroom and
> > the shared info in the buffer.
> > 
> > Currently the only way a 3K buffer would work is if FCoE is enabled and
> > in that case the driver is using order 1 pages and still using the
> > split buffer approach.
> 
> Hey Alex, interesting, we based this on the following logic from
> ixgbe_set_rx_buffer_len() I guess:
> 
> #if (PAGE_SIZE < 8192)
> 		if (adapter->flags2 & IXGBE_FLAG2_RSC_ENABLED)
> 			set_bit(__IXGBE_RX_3K_BUFFER, &rx_ring->state);
> 
> 		if (IXGBE_2K_TOO_SMALL_WITH_PADDING ||
> 		    (max_frame > (ETH_FRAME_LEN + ETH_FCS_LEN)))
> 			set_bit(__IXGBE_RX_3K_BUFFER, &rx_ring->state);
> #endif
> 
> so we assumed that ixgbe is no different than i40e/ice in these terms, but
> we ignored whole overhead of LRO/RSC that ixgbe carries.

If XDP is already enabled the LRO/RSC cannot be enabled. I think that
is already disabled if we have XDP enabled.

> I am not actively working with ixgbe but I know that you were the main dev
> of it, so without premature dive into the datasheet and codebase, are you
> really sure that 3k mtu for XDP is a no go?

I think I mixed up fm10k and ixgbe, either that or I was thinking of
the legacy setup. They all kind of blur together as I had worked on
pretty much all the Intel drivers up to i40e the last time I was
updating them for all the Rx path stuff. :)

So if I am reading things right the issue is that if XDP is enabled you
cannot set a 3K MTU, but if you set the 3K MTU first then you can
enable XDP after the fact right?

Looking it over again after re-reading the code this looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>