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

[Jakob Bohm]

On 04/01/2016 17:23, Prem Anand Haridoss wrote:
> 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?

I believe this is all happening in the PXE ROM running inside the
virtual machine and would be the same if the same version/brand of PXE
code was used on a real machine.  I don't know if all PXE ROMs on real
machines do this.


Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
This public discussion message is non-binding and may contain errors.
WiseMo - Remote Service Management for PCs, Phones and Embedded