Re: [Xen-users] ARM: Xen on Vexpress

[Julien Grall <julien.grall@linaro.org>]

On 18/06/14 09:29, Ian Campbell wrote:
> On Tue, 2014-06-17 at 18:50 +0100, Julien Grall wrote:
>> (CCing Xen-devel)
>>
>> On 06/16/2014 05:32 PM, Jeenu Viswambharan wrote:
>>> On Thu, Jun 12, 2014 at 15:50:02, Jeenu Viswambharan wrote:
>>>>>>    - What does DOM0 see as its PC at entry?
>>>>>
>>>>> The load address of the kernel image, since the zImage protocol
>>>>> requires us to enter the kernel at that offset.
>>>>
>>>> I managed to break at eret before guest entry, and the PC is
>>>> 0xafc00000 itself. It does fail to boot bare metal too from that
>>>> address. I'll see if I can find what's going wrong.
>>>
>>> I've been told that loading Linux at that high an offset from the memory
>>> base wouldn't work after all, even on bare metal. I therefore set
>>> kernel_addr_r to 0x80008000, which makes Xen load DOM0 at 0x8fc00000. I
>>> can see Linux starting to boot from this address on bare metal, but With
>>> Xen I'm seeing "uncompression error" (not printed on to the console, but
>>> through debugger inspection).
>>>
>>> I haven't pinned down why exactly it's throwing this. While I'm at it,
>>> I'd appreciate any helpful pointers.
>>
>> Did you try to use CONFIG_VMSPLIT_3G rather than CONFIG_VMSPLIT_2G as I
>> suggested last week?
>>
>> Using CONFIG_VMSPLIT_3G works correctly with 3.15. It looks like another
>> issue in the pa - virt delta when it's positive.
>>
>> IIRC, it's because linux is assuming that delta is always negative (see
>> comment on __PV_BITS_31_24 in arch/arm/include/asm/memory.h).
>>
>> At first glance, handling positive offset will be hard because, their
>> are rely on the compiler to provide the right layout for the
>> instruction. Positive number is not encoded the same way. I will have to
>> spend a bit more time to fully understand this code.
>>
>> With Ian multiple bank support patch for DOM0, I think we can safely
>> assume that the delta will (nearly?) always be the same.
>
> That would be a coincidence of the implementation, not a guarantee which
> we would make.
>
> If there is an issue with the pa - virt delta handling then we should
> raise it with Linux upstream.

I will raise again the issue to Linux. But I start to think this is a 
restriction by Linux but I can't find a clear comment about it. Not much 
(maybe none of) ARM platform has the start physical address very high.

Regards,

-- 
Julien Grall