The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

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The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

Brian Buhrow
        hello.  I'm trying to configure an hvm domain with a vnc listener  on
one of the dom0's network addresses, rather than 127.0.0.1.  

If I use a line like:
vfb = [ 'type=vnc,vnc=1,vnclisten=10.14.200.200' ]

in the config file, I still get a vnc listener on 127.0.0.1, rather than
10.14.200.200.

        A google search suggests this is somewhat of a known problem, but I
don't see a patch in the upstream version of xen-tools that addresses it.
Is there an easy fix for this issue, or perhaps I need to do what I want in
a different way?

-thanks
-Brian

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Re: The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

freebsd-xen mailing list
On Fri, Aug 28, 2020 at 08:47:41AM -0700, Brian Buhrow wrote:
> hello.  I'm trying to configure an hvm domain with a vnc listener  on
> one of the dom0's network addresses, rather than 127.0.0.1.  
>
> If I use a line like:
> vfb = [ 'type=vnc,vnc=1,vnclisten=10.14.200.200' ]

type=vnc doesn't seem to be a known option according to the xl.cfg man
page [0].

>
> in the config file, I still get a vnc listener on 127.0.0.1, rather than
> 10.14.200.200.

Do you also have a VNC sever without using vfb enabled? (ie: the
global xl.cfg vnc option). It might be useful if you post the full
xl.cfg maybe.

>
> A google search suggests this is somewhat of a known problem, but I
> don't see a patch in the upstream version of xen-tools that addresses it.
> Is there an easy fix for this issue, or perhaps I need to do what I want in
> a different way?

I think you will likely get more information by posting to xen-devel
directly, as this doesn't seem related to FreeBSD.

Roger.

[0] https://xenbits.xen.org/docs/4.12-testing/man/xl.cfg.5.html
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Re: The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

Brian Buhrow
        hello Roger.  thanks for the quick reply.  I think, after a careful
read of the xl.cfg man page plus a careful reading of the xen-tools source
code, I've figured out the issue.  I'm sending the solution here so folks
will have it in the future.

If one is configuring an hvm guest, any vfb specifications  in the domain
configuration are ignored.  Instead, the parameters that normally get placed
in a vfb stanza must be placed as top level configuration items.  For
example:

For a PV guest:
vfb = [ 'vnc=1,vnclisten=10.14.200.200' ]

Translates to, for an hvm guest:

vnc = 1
vnclisten = "10.14.200.200"

The xl.cfg man page doesn't say that in order for the vnc parameters to be
picked up for hvm guests with emulated graphics cards, those parameters
need to be specified as top level parameters in the config file.  It took
reading the source code of the xen-tools xl code to figure that out.

-thanks
-Brian

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Re: The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

freebsd-xen mailing list
On Fri, Aug 28, 2020 at 02:11:07PM -0700, Brian Buhrow wrote:
> hello Roger.  thanks for the quick reply.  I think, after a careful
> read of the xl.cfg man page plus a careful reading of the xen-tools source
> code, I've figured out the issue.  I'm sending the solution here so folks
> will have it in the future.
>
> If one is configuring an hvm guest, any vfb specifications  in the domain
> configuration are ignored.

Let me clarify this a bit. vfb is a para-virtualized frame buffer, so
it needs a specific Xen vfb driver in the guest for it to be able to
use the device (ie: it's not an emulated graphics card).

> Instead, the parameters that normally get placed
> in a vfb stanza must be placed as top level configuration items.  For
> example:
>
> For a PV guest:
> vfb = [ 'vnc=1,vnclisten=10.14.200.200' ]
>
> Translates to, for an hvm guest:
>
> vnc = 1
> vnclisten = "10.14.200.200"

OTOH, the global vnc parameter is indeed exclusive to HVM guests and
is related to the emulated graphics card device.

You could have a HVM guest making use of both the emulated graphics
card (the global vnc parameter) and nmultiple para-virtualized vfb
framebuffers, but those would be different outputs (like having more
than one graphics card on a physical box).

You cannot however make use of the global vnc parameter for PV guests,
as they have no emulated graphics card.

>
> The xl.cfg man page doesn't say that in order for the vnc parameters to be
> picked up for hvm guests with emulated graphics cards, those parameters
> need to be specified as top level parameters in the config file.  It took
> reading the source code of the xen-tools xl code to figure that out.

Sorry, I understand this is quite confusing. Do you think the man page
could be modified to make this easier to understand and create less
confusion?

I'm quite sure the community would be happy to take a patch in order
to clarify stuff.

Thanks, Roger.
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Re: The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

Brian Buhrow
        hello Roger.  Yes, I think an improvement to the xl.cfg man page would
be helpful.  Your additional notes clarifying my message are also useful in
helping to craft what that modified man page might look like.  In
particular, the section of the man page that says that vnc parameters wil
be picked up by hvm guests implies that they'll be picked up from within
the vfb spec.  that's simply not the case.  And, as you point out, it's
possible, though not probable, to have both an emulated graphics card on an
hvm guest and a pv graphics card on a hvm guest, both potentially sending
their vnc output to different IP addresses.  That's not at all clear from
the man page as I read it.
        I'll try to come up with some potential language and send it along to
you and this list.  If you think it helps, then it could be submitted as an
upstream patch.

-thanks
-Brian

On Aug 31,  9:55am, Roger Pau =?utf-8?B?TW9ubsOp?= wrote:
} Subject: Re: The vnclisten parameter is ignored with hvm domains under xen
} On Fri, Aug 28, 2020 at 02:11:07PM -0700, Brian Buhrow wrote:
} > hello Roger.  thanks for the quick reply.  I think, after a careful
} > read of the xl.cfg man page plus a careful reading of the xen-tools source
} > code, I've figured out the issue.  I'm sending the solution here so folks
} > will have it in the future.
} >
} > If one is configuring an hvm guest, any vfb specifications  in the domain
} > configuration are ignored.
}
} Let me clarify this a bit. vfb is a para-virtualized frame buffer, so
} it needs a specific Xen vfb driver in the guest for it to be able to
} use the device (ie: it's not an emulated graphics card).
}
} > Instead, the parameters that normally get placed
} > in a vfb stanza must be placed as top level configuration items.  For
} > example:
} >
} > For a PV guest:
} > vfb = [ 'vnc=1,vnclisten=10.14.200.200' ]
} >
} > Translates to, for an hvm guest:
} >
} > vnc = 1
} > vnclisten = "10.14.200.200"
}
} OTOH, the global vnc parameter is indeed exclusive to HVM guests and
} is related to the emulated graphics card device.
}
} You could have a HVM guest making use of both the emulated graphics
} card (the global vnc parameter) and nmultiple para-virtualized vfb
} framebuffers, but those would be different outputs (like having more
} than one graphics card on a physical box).
}
} You cannot however make use of the global vnc parameter for PV guests,
} as they have no emulated graphics card.
}
} >
} > The xl.cfg man page doesn't say that in order for the vnc parameters to be
} > picked up for hvm guests with emulated graphics cards, those parameters
} > need to be specified as top level parameters in the config file.  It took
} > reading the source code of the xen-tools xl code to figure that out.
}
} Sorry, I understand this is quite confusing. Do you think the man page
} could be modified to make this easier to understand and create less
} confusion?
}
} I'm quite sure the community would be happy to take a patch in order
} to clarify stuff.
}
} Thanks, Roger.
>-- End of excerpt from Roger Pau =?utf-8?B?TW9ubsOp?=


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Re: The vnclisten parameter is ignored with hvm domains under xen-4.12.1 and FreeBSD-12.0

freebsd-xen mailing list
On Mon, Aug 31, 2020 at 07:59:13AM -0700, Brian Buhrow wrote:

> hello Roger.  Yes, I think an improvement to the xl.cfg man page would
> be helpful.  Your additional notes clarifying my message are also useful in
> helping to craft what that modified man page might look like.  In
> particular, the section of the man page that says that vnc parameters wil
> be picked up by hvm guests implies that they'll be picked up from within
> the vfb spec.  that's simply not the case.  And, as you point out, it's
> possible, though not probable, to have both an emulated graphics card on an
> hvm guest and a pv graphics card on a hvm guest, both potentially sending
> their vnc output to different IP addresses.  That's not at all clear from
> the man page as I read it.
> I'll try to come up with some potential language and send it along to
> you and this list.  If you think it helps, then it could be submitted as an
> upstream patch.

Sure, I would certainly aim to have this upstream, as I don't carry
patches to the Xen packages unless they are backports from upstream
commits.

If you can provide the adjusted text I can prepare a patch and send
upstream, or feel free to do it yourself if you want to :).

Thanks, Roger.
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