Re: [Xen-users] qemu-system-i386 processes

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2015-01-08 at 12:19 +0100, "Sven MÃller" wrote:
> Hallo!
> 
> After upgrading from Xen 4.1 to 4.4 and switching from xm to xl as
> recommended, Xen 4.4 started one qemu-system-i386 process per Linux
> VM. Before upgrading, it was different, without qemu.
> 
> I am a little bit confused, are qemu-system-i386 processes nessassary
> for Linux VM? I am using filebased disk iamges.

As well as providing Device Models for HVM guests qemu can also provide
PV backends for all guest types, which includes PV FB, PV keyboard, PV
mouse and, as you are seeing here, PV disks ("qdisk").

Different toolstacks might choose a different disk backend depending on
the nature of the underlying disks. In particular newer Xen (can't
remember if it was 4.4 or 4.5) will prefer qemu's qdisk over tap disk
for more non-raw image types (so you may find you have traded a tapdisk
process for a qemu one).

xl also uses qdisk for raw disk images instead of loop+blkback which
xend used, because there are concerns that the loop driver can lead to
data loss (by not implementing direct i/o the underlying device, and/or
not handling flushes correct, my memory is a bit fuzzy). 

There were kernel patches for this at one point and some libxl patches
floating around to make it use loop+blkback, but I don't think those are
in 4.5 even.

For a PV guest the qemu process is only providing PV backends, it is not
doing any device emulation etc, which is where the big security concerns
would arise (and there is no path from the PV guest to that
functionality).

Ian.


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