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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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