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

From	Juergen Gross <jgross@suse.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page tables mapping
Date	2017-09-27 17:00 +0200
Message-ID	<uulYC-4wg-1@gated-at.bofh.it> (permalink)
References	<uuh8C-B0-15@gated-at.bofh.it> <uuh8C-B0-13@gated-at.bofh.it> <uukJb-3qZ-11@gated-at.bofh.it> <uulFg-4nX-23@gated-at.bofh.it> <uulOV-4rv-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

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

Thread

Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Juergen Gross <jgross@suse.com> - 2017-09-27 11:50 +0200
  Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-09-27 15:40 +0200
    Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Juergen Gross <jgross@suse.com> - 2017-09-27 16:40 +0200
      Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-09-27 16:50 +0200
        Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Juergen Gross <jgross@suse.com> - 2017-09-27 17:00 +0200
          Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-09-27 17:10 +0200
          Re: [Xen-devel] [PATCH v2] Call xen_cleanhighmap() with 4MB aligned  for page tables mapping Andrew Cooper <andrew.cooper3@citrix.com> - 2017-09-27 17:30 +0200
  Re: [PATCH v2] Call xen_cleanhighmap() with 4MB aligned for page  tables mapping Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-09-27 22:10 +0200

csiph-web