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

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Date:	Thu, 3 Sep 2015 15:45:07 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] xen/p2m: fix extra memory regions accounting

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.

See the use of PFN_DOWN() and PFN_UP() in xen_set_identify_and_remap().

David
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