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

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Date:	Thu, 3 Sep 2015 16:01:41 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>,
	Roger Pau Monné <roger.pau@...rix.com>,
	<linux-kernel@...r.kernel.org>
CC:	<xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen/p2m: fix extra memory regions accounting

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.

David
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