Re: [Xen-devel] Xen unstability on HP Moonshot m400

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 23 Mar 2015, Christoffer Dall wrote:
> On Mon, Mar 23, 2015 at 1:36 PM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
>       On Sat, 2015-03-21 at 13:34 +0100, Christoffer Dall wrote:
>       > Hi,
>       >
>       > I have been experiencing a problematic crash running Xen on m400 over
>       > the last few days. I already spoke to Ian and Stefano about this, but
>       > thought I'd summarize what I've seen so far and loop in a wider
>       > audience.
>       >
>       > The basic setup is this:
>       >Â - Two m400 nodes, one running Linux bare-metal, the other running
>       > Xen.
>       >Â - The Xen node runs Dom0 and 1 DomU
>       >Â - The m400 has a Mellanox Connectx-3 PCIe 10G ethernet card with two
>       > parts on it
>       >Â - Dom0 uses NAT forwarding from Dom0's eth0 (which is connected to
>       > the internet) and regular bridging to eth1 which is connected to a
>       > private VLAN to the bare-metal node
>       >Â - Dom0 and DomU are configured with 14GB of ram, 4 cpus each
>       >Â - DomU runs apache2 serving the GCC manual (see
>       > 
> https://github.com/chazy/kvmperf/blob/master/cmdline_tests/apache_install.sh)
>       >
>       > The bare-metal node runs apache bench, like this: "ab -n 100000 -c 100
>       
> >http://secure-web.cisco.com/1r5tZ8-7RF8gHRANwFdizEZzgeMsjxVO0yKbYiV4zy7LeiUfYBXMkFq7FGW_SZ1x-VxdzyK-ErDsOUiQ9z2x-N
> y7XkL_loHP8ene_BuNFscGyWmQ3r6CtXAYaZCY4xRmmPT1uJOsZDLMu7j-LfCOGmQDSdBwgW7QYukI2bCtTrXM/http%3A%2F%2F10.10.1.120%2F
>       gcc%2Findex.html"
>       >
>       > (10.10.1.120 is the DomU IP address of the bridged interface to eth1)
>       >
>       > What happens now is that the entire Xen node goes down. I see various
>       > errors in the kernel log, some examples:
>       > http://pastebin.ubuntu.com/10642148/
>       > http://pastebin.ubuntu.com/10642177/
>       > http://pastebin.ubuntu.com/10642181/
>       > http://pastebin.ubuntu.com/10635573/
>       >
>       >
>       > All Linux kernels are 3.18 plus some tweaks for the m400 cartridge:
>       > https://github.com/columbia/linux-kvm-arm/tree/columbia-armvirt-3.18
> 
>       Is it worth adding
>       
> https://git.kernel.org/cgit/linux/kernel/git/arm64/linux.git/commit/?id=285994a62c80f1d72c6924282bcb59608098d5ec
>       to your kernel? It isn't Xen specific but it's perhaps possible that 
> Xen opens the window wider.
> 
>       How confident are you in
>       
> https://github.com/columbia/linux-kvm-arm/commit/5e29cb0478f3d90e4f568d6bea6840960331bcbb
>  ?
>       (although I suppose you aren't running in ACPI mode if you are running
>       Xen?)
> 
> 
> I'm not confident at all, but Linux (last I checked was v3.19) doesn't boot 
> without it, so not sure if there's an
> alternative? Mark?

This patch is key: it doesn't look like it is setting
dev->archdata.dma_coherent appropriately, see the implementation of
set_arch_dma_coherent_ops.


> 
>       If we think the issue might be to do with coherency of foreign mappings
>       undergoing i/o from dom0 and we've already ruled out disk (by using a
>       loopback mounted rootfs) then it might be worth bodging netback to
>       always copy too.
> 
>       Adding a call to skb_orphan_frags right before the netif_receive_skb in
>       drivers/net/xen-netback/netback.c:xenvif_tx_submit is a simple but
>       rather inefficient way of doing that (so I hope it doesn't perturb the
>       issue).
> 
> 
> I'll be happy to try this.

If we are right and the problem is due to the commit above not setting
dma_coherent to true (the kernel will think that actually the network
card is not coherent), then Ian's workaround should hide the problem.


> 
>       Stefano (who is more familiar with the Linux swiotlb side of things than
>       me) is travelling this week so he'll be on West coast time, not sure
>       when he gets off a plane nor if he's on email anyway (he's at ELC + this
>       ARM ACPI thing)
> 
> 
> ok, we'll see what happens.
> 
> -Christoffer
> 
>

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