linux-kernel - Re: [Xen-devel] [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page tables mapping

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Date:   Wed, 27 Sep 2017 16:03:37 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>, <mingo@...hat.com>,
        <tglx@...utronix.de>, <hpa@...or.com>
CC:     <xen-devel@...ts.xenproject.org>, <x86@...nel.org>,
        <joe.jin@...cle.com>, <linux-kernel@...r.kernel.org>,
        <srinivas.eeda@...cle.com>
Subject: Re: [Xen-devel] [PATCH v2] Call xen_cleanhighmap() with 4MB aligned
 for page tables mapping

On 27/09/17 15:56, 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. :-)

Back in the day, superpages had 4M alignment.

~Andrew