linux-kernel - Re: Accessing more than 2GB of memory with a 32 bit kernel

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Message-ID: <CAE9FiQUxn7TzbcdnXpk7wXd=C0GX3myBS52BMJiDpdVVEdEquQ@mail.gmail.com>
Date:	Thu, 13 Jun 2013 17:31:53 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Simon Brown <smb@...c.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel

On Thu, Jun 13, 2013 at 5:14 PM, Rik van Riel <riel@...hat.com> wrote:
> On 06/13/2013 06:32 PM, Simon Brown wrote:
>>
>> On Wednesday 12 Jun 2013 16:10:03 Rik van Riel wrote:
>>>
>>> On 06/12/2013 12:54 PM, Simon Brown wrote:
>>>>
>>>> Hello,
>>>>
>>>> For the sake of an old prototype peripheral I'm using a non PAE 32 bit
>>>> x86 kernel and I'm having trouble accessing memory above 2 GB. The
>>>> system has 4GB installed and all is well with a PAE kernel.
>>>>
>>>> I'm obviously expecting to lose some memory due to memory mapped devices
>>>> but I wasn't expecting to lose 2GB. Instead I'm suspecting a BIOS bug.
>>>> The system reports:
>>>> free -m
>>>>
>>>>                total       used       free     shared    buffers
>>>>                cached
>>>>
>>>> Mem:          2012        491       1521          0         40
>>>> 277
>>>>
>>>> The mtrr table looked odd so I enabled sanitisation:
>>>> [    0.000000] original variable MTRRs
>>>> [    0.000000] reg 0, base: 2GB, range: 2GB, type UC
>>>> [    0.000000] reg 1, base: 0GB, range: 4GB, type WB
>>>> [    0.000000] reg 2, base: 4GB, range: 2GB, type WB
>>>> [    0.000000] total RAM covered: 4096M
>>>> [    0.000000] Found optimal setting for mtrr clean up
>>>> [    0.000000]  gran_size: 64K  chunk_size: 64K         num_reg: 2
>>>> lose cover RAM: 0G
>>>> [    0.000000] New variable MTRRs
>>>> [    0.000000] reg 0, base: 0GB, range: 2GB, type WB
>>>> [    0.000000] reg 1, base: 4GB, range: 2GB, type WB
>>>>
>>>> I don't understand the gap in the new table.
>>>
>>>
>>> Check the e820 table. Chances are the BIOS is reserving 2GB to
>>> map various devices (especially video cards) below the 4GB limit.
>>
>>
>> The table looks like this:
>>    [    0.000000] BIOS-provided physical RAM map:
>>    [    0.000000]  BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
>>    [    0.000000]  BIOS-e820: 000000000009fc00 - 00000000000a0000
>> (reserved)
>>    [    0.000000]  BIOS-e820: 00000000000e4000 - 0000000000100000
>> (reserved)
>>    [    0.000000]  BIOS-e820: 0000000000100000 - 000000007ff80000 (usable)
>>    [    0.000000]  BIOS-e820: 000000007ff80000 - 000000007ff8e000 (ACPI
>> data)
>>    [    0.000000]  BIOS-e820: 000000007ff8e000 - 000000007ffe0000 (ACPI
>> NVS)
>>    [    0.000000]  BIOS-e820: 000000007ffe0000 - 0000000080000000
>> (reserved)
>>    [    0.000000]  BIOS-e820: 00000000fee00000 - 00000000fee01000
>> (reserved)
>>    [    0.000000]  BIOS-e820: 00000000fff00000 - 0000000100000000
>> (reserved)
>>    [    0.000000]  BIOS-e820: 0000000100000000 - 0000000180000000 (usable)
>>
>> So the BIOS has reserved the entire upper half. Can I do anything about
>> that?
>
>
> Besides use a 64 bit kernel?  No.
>

or use 64bit kernel + kvm , qemu will have own e820 map

[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000dfffdfff] usable

so it looks like 3G ram under 4G.

but you need to make sure mb support vt-d/dmar, so you could use
pci_passthrough with your old prototype peripheral in your guest 32bit
kernel
without PAE.

Yinghai
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