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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 04/02/2016 23:14, Steven Haigh wrote:
> 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

Correct.

> - 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.

Incorrect.  A 32bit Qemu is perfectly able to emulate devices for a
64bit OS.

The ABI of the emulated devices is not tied to the running width of Qemu
or the OS.

>
> 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?
>

This will just be down to the choice of the person who packaged qemu for
a particular distro.

~Andrew