Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page tables mapping

[Juergen Gross]

On 27/09/17 16:48, Boris Ostrovsky wrote:
> On 09/27/2017 10:33 AM, Juergen Gross wrote:
>> On 27/09/17 15:38, Boris Ostrovsky wrote:
>>> On 09/27/2017 05:43 AM, Juergen Gross wrote:
>>>> On 27/09/17 11:41, Zhenzhong Duan wrote:
>>>>> When bootup a PVM guest with large memory(Ex.240GB), XEN provided initial
>>>>> mapping overlaps with kernel module virtual space. When mapping in this space
>>>>> is cleared by xen_cleanhighmap(), in certain case there could be an 2MB mapping
>>>>> left. This is due to XEN initialize 4MB aligned mapping but xen_cleanhighmap()
>>>>> finish at 2MB boundary.
>>> Does this mapping need to be 4MB-aligned?
>> I guess you are questioning the alignment of addr to be 4MB?
>> In this case you are right: the end of the mapping is 4MB aligned, as
>> correctly stated in the comment added.
> 
> Yes, and my question is why does it need to be aligned on 4MB. Doesn't
> 2MB alignment suffice?

I believe this has historical reasons. :-)

For this patch the answer doesn't matter, as Xen does it this way and
the kernel has to cope with the situation.

This interface is specified in include/xen/interface/xen.h in the
comment section just before struct start_info:

/*
 * Start-of-day memory layout
 *
 *  1. The domain is started within contiguous virtual-memory region.
 *  2. The contiguous region begins and ends on an aligned 4MB boundary.
...


Juergen