The 08/07/2019 09:06, Alexander Duyck wrote: > On Wed, Aug 7, 2019 at 7:09 AM Maciej Fijalkowski > <maciejromanfijalkow...@gmail.com> wrote: > > > > On Wed, 7 Aug 2019 08:38:43 +0000 > > Firo Yang <firo.y...@suse.com> wrote: > > > > > The 08/07/2019 15:56, Jacob Wen wrote: > > > > I think the description is not correct. Consider using something like > > > > below. > > > Thank you for comments. > > > > > > > > > > > In Xen environment, due to memory fragmentation ixgbe may allocate a > > > > 'DMA' > > > > buffer with pages that are not physically contiguous. > > > Actually, I didn't look into the reason why ixgbe got a DMA buffer which > > > was mapped to Xen-swiotlb area. > > > > I think that neither of these descriptions are telling us what was truly > > broken. They lack what Alexander wrote on v1 thread of this patch. > > > > ixgbe_dma_sync_frag is called only on case when the current descriptor has > > EOP > > bit set, skb was already allocated and you'll be adding a current buffer as > > a > > frag. DMA unmapping for the first frag was intentionally skipped and we > > will be > > unmapping it here, in ixgbe_dma_sync_frag. As Alex said, we're using the > > DMA_ATTR_SKIP_CPU_SYNC attribute which obliges us to perform a sync manually > > and it was missing. > > > > So IMHO the commit description should include descriptions from both xen and > > ixgbe worlds, the v2 lacks info about ixgbe case. Thank you. Will submit v3 with what Alexander worte on v1.
Thanks, Firo > > > > BTW Alex, what was the initial reason for holding off with unmapping the > > first > > buffer? Asking because IIRC the i40e and ice behave a bit different here. We > > don't look there for EOP at all when building/constructing skb and not > > delaying > > the unmap of non-eop buffers. > > > > Thanks, > > Maciej > > The reason why we have to hold off on unmapping the first buffer is > because in the case of Receive Side Coalescing (RSC), also known as Large > Receive Offload (LRO), the header of the packet is updated for each > additional frame that is added. As such you can end up with the device > writing data, header, data, header, data, header where each data write > would update a new descriptor, but the header will only ever update the > first descriptor in the chain. As such if we unmapped it earlier it would > result in an IOMMU fault because the device would be rewriting the header > after it had been unmapped. > > The devices supported by the ixgbe driver are the only ones that have > RSC/LRO support. As such this behavior is present for ixgbe, but not for > other Intel NIC drivers including igb, igbvf, ixgbevf, i40e, etc. > > - Alex >
