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

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Date:	Thu, 3 Sep 2015 16:38:25 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] xen/p2m: fix extra memory regions accounting

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).

Roger.
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