stubdomain questions

stubdomain questions

[Giorgenes Gelatti]

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Hi,

I've just ran an hvm with a stubdomain and I have a few questions. Would
someone answer it or point me where I could find the answers?

- what is the fs-backend used for?
- I saw that qemu-dm is still running. Isn't it the point of a stubdomain to
have qemu running inside the stubdomain instead of dom0?

tks in advance.

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Re: stubdomain questions

[Stefano Stabellini]

On Fri, 17 Jul 2009, Giorgenes Gelatti wrote:
> Hi,
> 
> I've just ran an hvm with a stubdomain and I have a few questions. Would someone answer it or point me where I could find
> the answers?
> 
> - what is the fs-backend used for?

At the moment it is only needed for save\restore, to read or write the
qemu statefile from dom0.

> - I saw that qemu-dm is still running. Isn't it the point of a stubdomain to have qemu running inside the stubdomain
> instead of dom0?
>

when using stubdomains, qemu in dom0 doesn't do device emulation but
provides few backends to the stubdom: the console backend and the pvfb
backend. The last one is needed to render the guest framebuffer using
sdl or for exporting it through vnc.

Re: stubdomain questions

[Giorgenes Gelatti]

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> when using stubdomains, qemu in dom0 doesn't do device emulation but
> provides few backends to the stubdom: the console backend and the pvfb
> backend. The last one is needed to render the guest framebuffer using
> sdl or for exporting it through vnc.
>

Is it possible (or is it being planned) to put these functions inside the
stubdomain too and completely eliminate the need of qemu-dm running in dom0?

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Re: stubdomain questions

[Stefano Stabellini]

On Fri, 17 Jul 2009, Giorgenes Gelatti wrote:
> 
>       when using stubdomains, qemu in dom0 doesn't do device emulation but
>       provides few backends to the stubdom: the console backend and the pvfb
>       backend. The last one is needed to render the guest framebuffer using
>       sdl or for exporting it through vnc.
> 
> 
> Is it possible (or is it being planned) to put these functions inside the stubdomain too and completely eliminate the
> need of qemu-dm running in dom0?
> 

No, this is not going to happen.
But qemu-dm in this case doesn't use many resources; if you are using
vnc and you don't have any client connected it is basically doing nothing,
otherwise just exporting the framebuffer or rendering to X11.
It is the same binary but this is the only thing in common between the
two qemu cases.

Re: stubdomain questions

[Samuel Thibault]

Giorgenes Gelatti, le Fri 17 Jul 2009 12:48:34 -0300, a écrit :
>     when using stubdomains, qemu in dom0 doesn't do device emulation but
>     provides few backends to the stubdom: the console backend and the pvfb
>     backend. The last one is needed to render the guest framebuffer using
>     sdl or for exporting it through vnc.
> 
> 
> Is it possible (or is it being planned) to put these functions inside the
> stubdomain too and completely eliminate the need of qemu-dm running in dom0?

In theory we could compile SDL into the stubdomain and let it connect to
the X server itself through TCP/IP.  That however would be very far from
efficient, as you loose memory-memory video blitting and go through
network links.

VNC server used to be possible in stubdomains, but the configuration is
quite cumbersome: you have to assign the stubdomain an IP. Letting dom0
do this was much more simple, especially since dirtyness tracking is
still done by the stubdomain, not dom0's ioemu.

Samuel

Re: stubdomain questions

[Giorgenes Gelatti]

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> > Is it possible (or is it being planned) to put these functions inside the
> > stubdomain too and completely eliminate the need of qemu-dm running in
> dom0?
>
> In theory we could compile SDL into the stubdomain and let it connect to
> the X server itself through TCP/IP.  That however would be very far from
> efficient, as you loose memory-memory video blitting and go through
> network links.
>
> VNC server used to be possible in stubdomains, but the configuration is
> quite cumbersome: you have to assign the stubdomain an IP. Letting dom0
> do this was much more simple, especially since dirtyness tracking is
> still done by the stubdomain, not dom0's ioemu.
>
> Samuel
>

So, in theory, if I didn't want VNC or SDL, and could like, have some video
passthrough, could I simply not run qemu-dm?

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Re: stubdomain questions

[Samuel Thibault]

Giorgenes Gelatti, le Fri 17 Jul 2009 13:00:53 -0300, a écrit :
> So, in theory, if I didn't want VNC or SDL, and could like, have some video
> passthrough, could I simply not run qemu-dm?

Yes.

Samuel

Re: stubdomain questions

[Giorgenes Gelatti]

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2009/7/17 Samuel Thibault <samuel.thibault@ens-lyon.org>

> Giorgenes Gelatti, le Fri 17 Jul 2009 13:00:53 -0300, a écrit :
> > So, in theory, if I didn't want VNC or SDL, and could like, have some
> video
> > passthrough, could I simply not run qemu-dm?
>
> Yes.
>
> Samuel
>


Could I run qemu-dm into another domain (domU)? Does it need any special
privileges?

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Re: stubdomain questions

[Samuel Thibault]

Giorgenes Gelatti, le Fri 17 Jul 2009 16:28:07 -0300, a écrit :
> 2009/7/17 Samuel Thibault <samuel.thibault@ens-lyon.org>
> 
>     Giorgenes Gelatti, le Fri 17 Jul 2009 13:00:53 -0300, a écrit :
>     > So, in theory, if I didn't want VNC or SDL, and could like, have some video
>     > passthrough, could I simply not run qemu-dm?
> 
> Could I run qemu-dm into another domain (domU)? Does it need any special
> privileges?

Mmm. It needs to be given the rights to connect to the fb frontend (i.e.
xenstore + memory pages), but else I guess it should be feasible.

Samuel

Re: stubdomain questions

[Samuel Thibault]

Giorgenes Gelatti, le Fri 17 Jul 2009 12:34:07 -0300, a écrit :
> - what is the fs-backend used for?

Mostly accessing qemu configuration files (like keyboard maps and stuff
like that).

> - I saw that qemu-dm is still running. Isn't it the point of a stubdomain to
> have qemu running inside the stubdomain instead of dom0?

You still need qemu-dm's SDL window or vnc server to show the screen of
the stubdomain emulating the VGA board.

Samuel