Re: [Xen-devel] [QUESTION] x86_64 -> i386/i686 CPU translation between xl and qemu binary?

[Steven Haigh <netwiz@xxxxxxxxx>]

On 2016-02-05 09:22, Andrew Cooper wrote:
> On 04/02/2016 22:06, Alex Braunegg wrote:
> > root     30511 46.4  0.1 398728  1860 ?        RLsl 08:47   0:27
> > /usr/lib/xen/bin/qemu-system-i386 -xen-domid 6 -chardev
> > socket,id=libxl-cmd,path=/var/run/xen/qmp-libxl-6,server,n
> > owait -no-shutdown -mon chardev=libxl-cmd,mode=control -chardev
> > socket,id=libxenstat-cmd,path=/var/run/xen/qmp-libxenstat-6,server,nowait
> > -mon chardev=libxenstat-cmd,mode=control
> > -nodefaults -name test2 -vnc
> > 0.0.0.0:0,websocket,x509=/etc/pki/xen,password,to=99 -display none 
> > -serial
> > pty -device VGA,vgamem_mb=16 -boot order=cd -usb -usbdevice tablet 
> > -soundhw
> >  ac97 -device rtl8139,id=nic0,netdev=net0,mac=00:16:3e:f1:48:8c 
> > -netdev
> > type=tap,id=net0,ifname=vif6.0-emu,script=no,downscript=no -machine 
> > xenfv -m
> > 496 -drive file=/dev/zvol/stor
> > age0/xen/test2/disk_sda,if=ide,index=0,media=disk,format=raw,cache=writeback
> > -drive
> > file=/storage0/data-shares/iso/CentOS-6.5-x86_64-minimal.iso,if=ide,index=2,
> > readonly=on,media=c
> > drom,format=raw,cache=writeback,id=ide-5632
> >
> > ------------------------------------------
> >
> > So - to me it appears that xl is performing some sort of x86_64 -> 
> > i386/i686
> > instruction translation to make things work.
> >
> > Would this not be introducing a performance impediment by having some 
> > sort
> > of extra translation processing going on between xl and the qemu 
> > binary?
> Qemu is only used for device emulation when used with Xen, not CPU
> emulation.
>
> The "-machine xenfv" tells this to Qemu, and "-xen-domid 6" tells it
> which Xen domain to connect to.
>
> All HVM domains run with hardware virtualisation extensions, which are
> managed by Xen itself.
Hi Andrew,

Thanks for this response - to ensure I have this correct, there is no 
need for qemu-upstream to build qemu-system-x86_64 as the CPU is 
directly handled by Xen and not by qemu - thereby passing through the 
capabilities of the CPU directly to the guest. As such, as long as qemu 
starts on a 64 bit machine it will be able to run 64 bit OS/kernel etc.
I ask this as I see a number of qemu packages that do include 
qemu-system-x86_64 as well as qemu-system-i386 - which makes me seek 
clarification. I would assume that these are just not built to use Xen 
as the hypervisor for hardware acceleration?
--
Steven Haigh

Email: netwiz@xxxxxxxxx
Web: https://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897

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