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

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Message-ID: <55E85F52.9030708@suse.com>
Date:	Thu, 3 Sep 2015 16:55:14 +0200
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.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 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.


Juergen

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