Re: [Xen-devel] segfault in VM

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> As a first test I have just disabled networking via nics=0 in the config, and 
> running this script in dom1:
> #!/bin/sh
> while [ 1 = 1 ]
> do
>   dd if=/dev/sda1 of=/dev/null bs=1024 count=128K &
>   dd if=/dev/sda1 of=/dev/null bs=1024 skip=256K count=256K
> done
> 
> it tells me 'ioctl 801c6d02 not supported by XL blkif' but that doesn't seem 
> to matter. Anyway, there are no crashes so far so i'm thinking at this stage 
> that the block interface stuff is probably fine and I should now concentrate 
> on the network. Disabling the block stuff will be a huge hassle at this stage 
> so i'll have to let it go for the moment.

It does seem more likely that the network backend driver is to blame
-- it's considerably more complicated than the blkdev driver.

> I think i need a crash course in how all this hangs together before I can 
> understand what i'm testing... My understanding is as follows:
> 
> packets sent to dom0.vif1.0 appear at dom1.eth0.
> packets sent to dom1.eth0 appear at dom0.vif1.0.

Yes, it's basically a point-to-point link. The transmit side on each
interface is directly linked to the receive side on the other.

> and that's about it. Are they symmetrical? Is the transmit code for 
> dom0.vif1.0 the same as the transmit code for dom1.eth0? Ditto for receive?

No. dom1.eth0 is implemented by the frontend driver
arch/xen/drivers/netif/frontend/main.c
dom0.vif* is implemented by arch/xen/drivers/netif/backend/main.c 

So they look symmetric to users, but the implementation is not
symmetric. 

 -- Keir


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