Re: [Xen-users] Debugging DomU

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 14/05/2015 20:06, Chris (Christopher) Brand wrote:
> Hi Julien,
Hi Christopher,

> > This is normal that address 0 is not mapped to any RAM. The memory layout of 
> > the guest (see xen/include/public/arch-arm.h) as the RAM guest defined in 
> > higher memory.
> Ok. That seems to match what I'm seeing in the MMU - VTTBR for that guest 
> contains two L1 tables, one at 0 and one at 0x40000000 (GUEST_RAM_BASE). The 
> first is almost empty while the second has quite a few mappings. I assume VTTBR 
> is where I should be looking ?
>
> > Although, vmlinux is using virtual address, so 0 is likely a virtual address. 
> > Do you know if the page table has been setup to the guest?
> I see a call to p2m_alloc_table(), followed by a number of calls to 
> p2m_create_table().
p2m_alloc_table() and p2m_create_table() are used in Xen for creating 
the stage-2 page table. I was asking if the kernel already setup his PT 
and enable pagination.
> > The error you describe in your mail is usually happening in the early setup.
> That seems to be the case, yes. I haven't been able to see anything that's 
> happening in the VM before this.
>
> > I don't think this is the bug, as said above we don't map anything to address 0 
> > and this is normal because it's not part of the layout.
> Ok. It certainly wouldn't be surprising if I'm getting my address types mixed 
> up :-)
>
> > It would be useful if you provide the version of Xen and Linux you use.
> Xen is a recent git clone - cb34a7c8d741, with a couple of small hacks that we 
> needed to get to the point of Dom0 running on our platform (there's a change to 
> xen/drivers/char/ns16550.c to workaround a uart bug, and one to 
> xen/arch/platforms/brcm.c to set dom0_gnttab_start and dom0_gnttab_end).
> Linux Kernel is 3.14. I took our kernel, enabled the various CONFIG options for 
> Xen and rebuilt it.
The important CONFIG_* that should be enable are:
        - CONFIG_XEN_*
        - CONFIG_ARCH_MULTIPLATFORM (required to enable the next option)
        - CONFIG_ARCH_VIRT
        - CONFIG_AUTO_ZRELADDR
        - CONFIG_ARM_PATCH_PHYS_VIRT

I would also advice you to backport the commit 
238962ac71910d6c20162ea5230685fead1836a4 "ARM: 8191/1: decompressor: 
ensure I-side picks up relocated code". It may help to boot further as 
it fixes an issue with similar symptoms as yours.
Regards,

--
Julien Grall

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