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

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Message-ID: <55E85EB6.7060307@citrix.com>
Date:	Thu, 3 Sep 2015 15:52:38 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Roger Pau Monné <roger.pau@...rix.com>,
	Juergen Gross <jgross@...e.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 15:45, 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).

David
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