On Wed, 18 Mar 2020 14:23:09 -0700 Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote: > On Wed, Mar 18, 2020 at 1:04 PM Maciej Fijalkowski > <maciej.fijalkowski@xxxxxxxxx> 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? -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
