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Date: Fri, 28 Oct 2016 08:56:35 -0700 From: John Fastabend <john.fastabend@...il.com> To: David Miller <davem@...emloft.net>, alexander.duyck@...il.com Cc: mst@...hat.com, brouer@...hat.com, shrijeet@...il.com, tom@...bertland.com, netdev@...r.kernel.org, shm@...ulusnetworks.com, roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com Subject: Re: [PATCH net-next RFC WIP] Patch for XDP support for virtio_net On 16-10-27 07:10 PM, David Miller wrote: > From: Alexander Duyck <alexander.duyck@...il.com> > Date: Thu, 27 Oct 2016 18:43:59 -0700 > >> On Thu, Oct 27, 2016 at 6:35 PM, David Miller <davem@...emloft.net> wrote: >>> From: "Michael S. Tsirkin" <mst@...hat.com> >>> Date: Fri, 28 Oct 2016 01:25:48 +0300 >>> >>>> On Thu, Oct 27, 2016 at 05:42:18PM -0400, David Miller wrote: >>>>> From: "Michael S. Tsirkin" <mst@...hat.com> >>>>> Date: Fri, 28 Oct 2016 00:30:35 +0300 >>>>> >>>>>> Something I'd like to understand is how does XDP address the >>>>>> problem that 100Byte packets are consuming 4K of memory now. >>>>> >>>>> Via page pools. We're going to make a generic one, but right now >>>>> each and every driver implements a quick list of pages to allocate >>>>> from (and thus avoid the DMA man/unmap overhead, etc.) >>>> >>>> So to clarify, ATM virtio doesn't attempt to avoid dma map/unmap >>>> so there should be no issue with that even when using sub/page >>>> regions, assuming DMA APIs support sub-page map/unmap correctly. >>> >>> That's not what I said. >>> >>> The page pools are meant to address the performance degradation from >>> going to having one packet per page for the sake of XDP's >>> requirements. >>> >>> You still need to have one packet per page for correct XDP operation >>> whether you do page pools or not, and whether you have DMA mapping >>> (or it's equivalent virutalization operation) or not. >> >> Maybe I am missing something here, but why do you need to limit things >> to one packet per page for correct XDP operation? Most of the drivers >> out there now are usually storing something closer to at least 2 >> packets per page, and with the DMA API fixes I am working on there >> should be no issue with changing the contents inside those pages since >> we won't invalidate or overwrite the data after the DMA buffer has >> been synchronized for use by the CPU. > > Because with SKB's you can share the page with other packets. > > With XDP you simply cannot. > > It's software semantics that are the issue. SKB frag list pages > are read only, XDP packets are writable. > > This has nothing to do with "writability" of the pages wrt. DMA > mapping or cpu mappings. > Sorry I'm not seeing it either. The current xdp_buff is defined by, struct xdp_buff { void *data; void *data_end; }; The verifier has an xdp_is_valid_access() check to ensure we don't go past data_end. The page for now at least never leaves the driver. For the work to get xmit to other devices working I'm still not sure I see any issue. .John
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