Xen devel - Mar 2013 - What's the benefit and design purpose of Xen Console for HVM guest.

Hi guys,
I''m investigating Xen Console recently, i take some test on
Linux(Fedora)
hvm guest, it works good. (Xen-4.1.3 and Linux 3.5.0)

As far as i know, the performance improvement of Xen Console seems to be
great, but it looks like it just provides a shell for login via Xen Console
for function concern, since it''s a console,
i wonder if it is designed to be a system console? or just an additional
shell access to the guest like ssh(rlogin),  that means is it able to
replace the emulated serial by qemu-dm totally, can it be able to print the
early boot message and debugging message generated by printk() like
function?

Any answer would be appreciated a lot, thanks.

Regards,
Wei


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

On Fri, 29 Mar 2013, Wei Xu wrote:
> Hi guys,
> I''m investigating Xen Console recently, i take some test on
Linux(Fedora) hvm guest, it works good. (Xen-4.1.3 and Linux 3.5.0)
> 
> As far as i know, the performance improvement of Xen Console seems to be
great, but it looks like it just provides a shell for
> login via Xen Console for function concern, since it''s a console,
> i wonder if it is designed to be a system console? or just an additional
shell access to the guest like ssh(rlogin),  that means
> is it able to replace the emulated serial by qemu-dm totally, can it be
able to print the early boot message and debugging
> message generated by printk() like function?  
Yes, you just need to pass console=hvc0 to the kernel command line.
It''s not just useful for debugging, it''s also possible to add
a large
number of PV consoles to an HVM guest, and they should all work without
issues: one could be only used for logging kernel messages for example,
another one for logging something else, and the rest as login shells.
At the moment xl doesn''t export a config option to do that, but libxl
knows how to create as many PV console as you like.
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

CC''ing back xen-devel.

On Fri, 29 Mar 2013, Wei Xu wrote:
> I have test the kgdb kernel debugging via serial, it works good when i used
the physical serial to connect the 2 HVM guests(host
> and target), is it possible to debugging kernel via Xen Console?
I know very little about kgdb but I know that other people on the list
use it. I guess it should be possible.

> I know xen console will related to different file in "/dev/pts"
for different HVM guest, so this is impossible by now, is it
> possible to fit it?  that means xen console is a full replacement of the
emulated serial.
I don''t understand what you mean here. Xen PV consoles also correspond
to
some /dev/pts devices in dom0.

> what i am wondering most is why did Xen Console come to mind in the early
date, that''s why did it raise? Was it going to replace
> the emulated serial, or just an additional way to access the guest with
high performance or sth other.
Two main reasons come to mind: if you need additional console for
logging, PV consoles provide you with a solution.
If you need to debug an PV on HVM kernel very early on at resume time,
PV consoles can be re-initialized much earlier than emulated serials.
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

thanks, comments inlined.

On Sat, Mar 30, 2013 at 12:30 AM, Stefano Stabellini <
stefano.stabellini@eu.citrix.com> wrote:
> CC''ing back xen-devel.
>
> On Fri, 29 Mar 2013, Wei Xu wrote:
> > I have test the kgdb kernel debugging via serial, it works good when i
> used the physical serial to connect the 2 HVM guests(host
> > and target), is it possible to debugging kernel via Xen Console?
>
> I know very little about kgdb but I know that other people on the list
> use it. I guess it should be possible.
>
>
> > I know xen console will related to different file in
"/dev/pts" for
> different HVM guest, so this is impossible by now, is it
> > possible to fit it?  that means xen console is a full replacement of
the
> emulated serial.
>
> I don''t understand what you mean here. Xen PV consoles also
correspond to
> some /dev/pts devices in dom0.
>
>
yes, i know, kgdb need a pipe-like serial pair to pass data between the 2
guests, while xenconsoled tells the PV console was linked to different
files in /dev/pts bundled with only one guest, so i don''t know if it is
possible to link/pipe them together.

 > what i am wondering most is why did Xen Console come to mind in
the
> early date, that''s why did it raise? Was it going to replace
> > the emulated serial, or just an additional way to access the guest
with
> high performance or sth other.
>
> Two main reasons come to mind: if you need additional console for
> logging, PV consoles provide you with a solution.
> If you need to debug an PV on HVM kernel very early on at resume time,
> PV consoles can be re-initialized much earlier than emulated serials.
Why emulated serial can''t be re-initialized during resume time?
i''m a
little confused.
beyond the 2 reasons, why performance improvement isn''t the main
concern?
or some other reasones unmentioned here?


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel