Date: Fri, 5 Sep 2014 15:05:34 -0400 From: Jeremiah Lott <jlott@averesystems.com> To: freebsd-xen@freebsd.org Subject: netfront driver with mbufs > 4K Message-ID: <CANG7ib9TQz3%2Bk2ubMqtp0_YwRVi-96U8ZSr%2BFyRrC5tmqfuMCQ@mail.gmail.com>
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It seems the netback driver (at least in linux) cannot handle a fragment passed to it that crosses a 4K boundary. Here is the code from the linux netback driver that enforces this: if (unlikely((txp->offset + txp->size) > PAGE_SIZE)) { netdev_err(vif->dev, "Cross page boundary, txp->offset: %x, size: %u\n", txp->offset, txp->size); xenvif_fatal_tx_err(vif); return -EINVAL; } The netfront driver in FreeBSD creates a fragment for each mbuf in the chain, but does not check if the mbuf crosses a 4K boundary. For mbufs allocated from the zone_jumbo9 or zone_jumbo16 uma zones, it can create fragments that are too large and/or cross a page boundary. In this case the netback stops responding, and the network interface appears to "hang". It is also a problem for mbufs that have external data attached to them (as the data could be any size or alignment). It seems that the netfront driver needs split mbufs that span page boundaries into multiple fragments. I'm starting to think about code to do that, but I figured I'd put this out there if anyone has suggestions on handling this. Thanks, Jeremiah Lott Avere Systems
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