Subject: Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page
 tables mapping
To: Juergen Gross <jgross@suse.com>,
        Zhenzhong Duan <zhenzhong.duan@oracle.com>, mingo@redhat.com,
        tglx@linutronix.de, hpa@zytor.com
References: <85bd42d5-b0d2-40f5-81a9-14cb51ec4503@default>
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 <44ebc914-d1a3-06ac-9ec5-b866d5f04e0a@oracle.com>
 <9faf8d7b-227f-2ecd-26e6-44ae4270d469@suse.com>
 <df140683-63dd-02fa-f248-2fcb6d635d54@oracle.com>
 <30c5a1f0-4425-6c8b-5017-a78710c0bd72@suse.com>
Cc: x86@kernel.org, xen-devel@lists.xenproject.org,
        srinivas.eeda@oracle.com, joe.jin@oracle.com,
        linux-kernel@vger.kernel.org
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <64892ee3-4b6d-acd6-dbf5-dd1e84b25ce3@oracle.com>
Date: Wed, 27 Sep 2017 11:02:33 -0400
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On 09/27/2017 10:56 AM, Juergen Gross wrote:
> 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.

Ah, this is what I was really looking for --- that 4MB alignment is part
of the ABI.

-boris