Re: [Xen-devel] [PATCH] libxc/arm: Correctly handle the difference between virtual and physical address

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 12/11/2013 10:02 AM, Ian Campbell wrote:
> On Tue, 2013-12-10 at 18:16 +0000, Julien Grall wrote:
>> This issue is when as virt == phys, build ELF with virt != phys is very 
>> difficult.
> 
> OK, so I think I was mislead by your commit message, this is not
> actually about virt vs phys as such (or as represented in the ELF
> header) but is actually more about link address vs. load address. Where
> link address == ELF vaddr (==paddr). And because the MMU is disable load
> address == some vaddr (==paddr), which may differ from the ELF vaddr.
> 
> The terminology in xc_dom, which uses vaddr a lot because of PV x86,
> isn't helpful here but is that an accurate summary?

Right, and this address is actually equal to ELF vaddr. But the device
tree base address will be computed with the physical address.

> [...]
>> When the guest is creating, the ELF should loaded like zImage at the specific
>> physical address.
> 
> So that would be my next question -- where does this load address come
> from?

Load address ? Virtual ? Physical ?

> I suppose it has to be the 0x80100000 kernel physical address you gave
> earlier?
> 
> I think this means "where is dom->parms.virt_base" initialised. Have you
> added some ELF notes to your BSD kernel image?

I have tried to play with the ELF notes. My current configuration is:

XEN_ELFNOTE_VIRT_BASE    vaddr          (0xC0000000)
XEN_ELFNOTE_PADDR_OFFSET vaddr          (0xC0000000)

It's copied from Linux.

If I try to modify PADDR_OFFSET with (KERNBASE - PHYSADDR) or whatever
value it doesn't work. I got
"segment kernel too large (0x355 > 0x4000 - 0x80100 pages): Out of memory".

>>  Then the guest will start will MMU turn off, and during the
>> first instructions it will use fixup to get the right address.
> 
> "fixup" == enable the MMU, or "fixup" == some sort of relocation/PIC?

Some sort of relocation. When MMU is turn off, every time the kernel
needs to call asm callback, it will add an offset.

-- 
Julien Grall

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