linux-kernel - Re: [Xen-devel] [PATCH] xen/p2m: fix extra memory regions accounting

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Date:	Fri, 4 Sep 2015 07:07:17 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen/p2m: fix extra memory regions accounting

Hi Roger,

On 09/03/2015 05:39 PM, Roger Pau Monné wrote:
> El 03/09/15 a les 17.20, Juergen Gross ha escrit:
>> On 09/03/2015 05:01 PM, David Vrabel wrote:
>>> On 03/09/15 15:55, Juergen Gross wrote:
>>>> On 09/03/2015 04:52 PM, David Vrabel wrote:
>>>>> On 03/09/15 15:45, David Vrabel wrote:
>>>>>> On 03/09/15 15:38, Roger Pau Monné wrote:
>>>>>>> El 03/09/15 a les 14.25, Juergen Gross ha escrit:
>>>>>>>> On 09/03/2015 02:05 PM, Roger Pau Monne wrote:
>>>>>>>>> On systems with memory maps with ranges that don't end at page
>>>>>>>>> boundaries,
>>>>>>>>> like:
>>>>>>>>>
>>>>>>>>> [...]
>>>>>>>>> (XEN)  0000000000100000 - 00000000dfdf9c00 (usable)
>>>>>>>>> (XEN)  00000000dfdf9c00 - 00000000dfe4bc00 (ACPI NVS)
>>>>>>>>> [...]
>>>>>>>>>
>>>>>>>>> xen_add_extra_mem will create a protected range that ends up at
>>>>>>>>> 0xdfdf9c00,
>>>>>>>>> but the function used to check if a memory address is inside of a
>>>>>>>>> protected
>>>>>>>>> range works with pfns, which means that an attempt to map
>>>>>>>>> 0xdfdf9c00
>>>>>>>>> will be
>>>>>>>>> refused because the check is performed against 0xdfdf9000
>>>>>>>>> instead of
>>>>>>>>> 0xdfdf9c00.
>>>>>>>>>
>>>>>>>>> In order to fix this, make sure that the ranges that are added
>>>>>>>>> to the
>>>>>>>>> xen_extra_mem array are aligned to page boundaries.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
>>>>>>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>>>>>>>>> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>>>>>>>> Cc: David Vrabel <david.vrabel@...rix.com>
>>>>>>>>> Cc: Juergen Gross <jgross@...e.com>
>>>>>>>>> Cc: xen-devel@...ts.xenproject.org
>>>>>>>>> ---
>>>>>>>>> AFAICT this patch needs to be backported to 3.19, 4.0, 4.1 and 4.2.
>>>>>>>>> ---
>>>>>>>>>      arch/x86/xen/setup.c | 6 ++++++
>>>>>>>>>      1 file changed, 6 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
>>>>>>>>> index 55f388e..dcf5865 100644
>>>>>>>>> --- a/arch/x86/xen/setup.c
>>>>>>>>> +++ b/arch/x86/xen/setup.c
>>>>>>>>> @@ -68,6 +68,9 @@ static void __init xen_add_extra_mem(phys_addr_t
>>>>>>>>> start, phys_addr_t size)
>>>>>>>>>      {
>>>>>>>>>          int i;
>>>>>>>>>
>>>>>>>>> +    start = PAGE_ALIGN(start);
>>>>>>>>> +    size &= PAGE_MASK;
>>>>>>>>
>>>>>>>> This is not correct. If start wasn't page aligned and size was,
>>>>>>>> you'll
>>>>>>>> add one additional page to xen_extra_mem.
>>>>>>>
>>>>>>> I'm not understanding this, let's put an example:
>>>>>>>
>>>>>>> start = 0x8c00
>>>>>>> size = 0x1000
>>>>>>>
>>>>>>> After the fixup added above this would become:
>>>>>>>
>>>>>>> start = 0x9000
>>>>>>> size = 0x1000
>>>>>>>
>>>>>>> So if anything, I'm adding one page less (because 0x8000 was partly
>>>>>>> added, and with the fixup it is not added).
>>>>>>
>>>>>> We expand the reserved (i.e., non-RAM) areas down so they're fully
>>>>>> covered with whole pages when we depopulate and 1:1 map them, we
>>>>>> should
>>>>>> add extra memory regions that cover these same areas.
>>>>>
>>>>> Ignore this.  This was nonsense.
>>>>>
>>>>> We expand the reserved (i.e., non-RAM) areas so they're fully covered
>>>>> with whole pages when we depopulate and 1:1 map them, we should add the
>>>>> extra memory such that it does not overlap with with expanded regions.
>>>>> i.e., round up the start and round down the end (like Roger's patch
>>>>> does).
>>>>
>>>> Nearly. Roger's patch rounds up start and rounds down the size. It might
>>>> add non-RAM partial pages to xen_extra_mem.
>>>
>>> Yes.  You're right.
>>
>> Hmm, thinking more about it, I'd prefer to change xen_extra_mem to use
>> pfns instead of physical addresses. This would make things much more
>> clear.
>>
>> Roger, do you want to do the patch or should I?
>
> I can certainly take care of it if you are busy, otherwise I leave it to
> you since you have more expertise on it :).

Could you try the attached patch? It should do the job. It is booting
fine on my laptop, but I think you should try it on the machine with
the memory ranges not at page boundary.


Juergen


View attachment "extramem.patch" of type "text/x-patch" (6690 bytes)