Re: [Xen-devel] [PATCH 1/2] libxl: set stdvga=1 by default when creating a hvm guest

["Ren, Yongjie" <yongjie.ren@xxxxxxxxx>]

> -----Original Message-----
> From: Stefano Stabellini [mailto:stefano.stabellini@xxxxxxxxxxxxx]
> Sent: Wednesday, June 27, 2012 1:00 AM
> To: Ian Campbell
> Cc: Ren, Yongjie; xen-devel@xxxxxxxxxxxxx; Ian Jackson; Tim (Xen.org);
> Stefano Stabellini
> Subject: Re: [PATCH 1/2] libxl: set stdvga=1 by default when creating a hvm
> guest
> 
> On Tue, 26 Jun 2012, Ian Campbell wrote:
> > On Tue, 2012-06-26 at 07:02 +0100, Ren, Yongjie wrote:
> > > libxl: set stdvga=1 by default when creating a hvm guest
> > >
> > > Most of the modern OSes (e.g. Windows XP, Windows 7, RHEL6.x,
> Ubuntu, Fedora) support VBE 2.0 or later.
> > > So, select a standard VGA card with VBE as the default emulated
> graphics device.
> >
> > I'm not expert on the graphics side of HVM, but on the face of it
> > switching the default to something more modern seems like a
> reasonable
> > idea, although I'm not sure if we should be doing this for 4.2 at this
> > point.
> >
> > I've CCd Tim and Stefano for input from the HVM and QEMU sides.
> 
> I think it is a good thing.
> The only thing to keep in mind is that QEMU upstream is switching to
> 16MB of videoram for stdvga. So at some point in the near future
> upstream QEMU will stop working correctly with xen 4.2, unless we bump
> the videoram to 16MB too.
> 
Yes, we should pay attention to this when using upstream QEMU.

> 
> > > It's also a workaround for the following bug.
> > > http://bugzilla.xen.org/bugzilla/show_bug.cgi?id=1812
> >
> > Do you understand the root cause of that bug?
> >
I don't understand the root cause.

> > It's hard to see how detaching a VF relates to the VGA emulation in use.
> > Can you explain it? Are you sure you aren't just masking the real issue
> > here?
> 
It's strange that detaching a VF may break the graphics display.
As it only happens when 'stdvga=0', it might not be a normal usage.

> Indeed. We cannot possibly accept the patch on the basis that it looks
> like it is masking an unrelated pci-passthrough bug.
>
I don't want to mask that bug, either. :-)
The following sentence is quoted from xl.cfg man page.
"If your guest supports VBE 2.0 or later (e.g. Windows XP onwards) 
then you should enable this (stdvga option)."
If we set 'stdvga=1', we will not meet the bug (#1812).
I assume many Xen users (including me) are not very familiar with 'stdvga' and 
will leave it as default (it's 0 before my patch). 
If then, users may meet something *strange* like bug #1812.
I don't think it's friendly to end users, so I set the default value of 
'stdvga' to '1'.


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