Re: [Qemu-discuss] Qemu reports slightly less memory on PXE boot

[Prem Anand Haridoss]

From: address@hidden [mailto:address@hidden On Behalf Of Jakob Bohm
Sent: 04 January 2016 15:56
To: address@hidden
Subject: Re: [Qemu-discuss] Qemu reports slightly less memory on PXE boot

>> On 04/01/2016 15:21, Prem Anand Haridoss wrote:
>> On booting an iso (from the cdrom) with 8G memory, after bootup, I see 
>> 8179816 kB as the total memory available.
>> However on PXE booting the same vCPE livecd iso over a nic, with the same 8G 
>> memory, we see only 7857512 kB as the total memory available
>> Is this a bug in qemu? I am using version 2.5.0 (Debian 1:2.5+dfsg-1)
>> Regards
>> Prem
>> 
>> Logs:
>> =====
>> On booting iso using cdrom
>> --------------------------
>> address@hidden:~/work/tplaf/☸ qemu-system-x86_64 -name Test -nographic 
>> -enable-kvm -cpu Haswell -smp 2 -m 8G -cdrom $iso -boot d
>> 
>> ISOLINUX 4.05 20121210  Copyright (C) 1994-2011 H. Peter Anvin et al
>> Press control and F then 1 for help, or ENTER to
>> boot: live-serial init=/bin/bash
>> Loading, please wait...
>> bash: no job control in this shell
>> root@(none):/#
>> root@(none):/# cat /proc/meminfo | head -n3
>> MemTotal:        8179816 kB
>> MemFree:         8138960 kB
>> MemAvailable:    8131960 kB
>> 
>> On pxe booting the iso over a nic
>> ---------------------------------
>> address@hidden:~/☸ qemu-system-x86_64 -name Test -nographic -enable-kvm -cpu 
>> Haswell -smp 2 -m 8G -netdev 
>> user,id=net0,tftp=$HOME/TPLAF//TB2/rtr2/tftp,bootfile=/gpxelinux.0 -device 
>> rtl8139,netdev=net0 -boot once=n                       >>
>> PXELINUX 6.03 PXE 6.03-pre20 Copyright (C) 1994-2014 H. Peter Anvin et al
>> boot: Build109.11
>> Loading memdisk... ok
>> Loading <deleted>...ok
>>
>> ISOLINUX 4.05 20121210  Copyright (C) 1994-2011 H. Peter Anvin et al
>> Press control and F then 1 for help, or ENTER to
>> boot: live-serial init=/bin/bash
>> Loading, please wait...
>> bash: no job control in this shell
>> root@(none):/#
>> root@(none):/# cat /proc/meminfo | head -n3
>> MemTotal:        7857512 kB
>> MemFree:         7816764 kB
>> MemAvailable:    7809988 kB
>> Presumably, the PXE BIOS needs some RAM for the network
>> stack and CD-ROM emulation.
>>
> Perhaps it simply loads the entire ISO image into RAM 
> before booting it, so the boot process can play with 
> the network adapter without loosing access to its "CD".
>
> Check if the difference is close to the size of the ISO 
> file.
> 
Thanks Jakob
You are right!
The difference in memory matches the iso file size. So it seems that qemu uses 
certain portion of ram for storing the iso
But shouldn't qemu allocate extra memory needed to store the iso instead of 
using the ram specified to the guest OS. Or is this how a real device would 
behave when we PXE boot?

Regards
Prem

>
>
> Enjoy
>
> Jakob
> -- 
> Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
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