Re: [Xen-devel] [libvirt] [PATCH v2] libxl: Implement basic video device selection

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Fri, Apr 04, 2014 at 11:34:17AM +0100, Ian Campbell wrote:
> On Fri, 2014-04-04 at 12:31 +0200, Stefan Bader wrote:
> > On 04.04.2014 11:48, Ian Campbell wrote:
> > > On Fri, 2014-04-04 at 11:36 +0200, Stefan Bader wrote:
> > >> +    /*
> > >> +     * Take the first defined video device (graphics card) to display
> > >> +     * on the first graphics device (display).
> > >> +     * Right now only type and vram info is used and anything beside
> > >> +     * type xen and vga is mapped to cirrus.
> > >> +     */
> > >> +    if (def->nvideos) {
> > >> +        unsigned int min_vram = 8 * 1024;
> > >> +
> > >> +        switch (def->videos[0]->type) {
> > >> +            case VIR_DOMAIN_VIDEO_TYPE_VGA:
> > >> +            case VIR_DOMAIN_VIDEO_TYPE_XEN:
> > >> +                b_info->u.hvm.vga.kind = LIBXL_VGA_INTERFACE_TYPE_STD;
> > >> +                /*
> > >> +                 * Libxl enforces a minimal VRAM size of 8M when using
> > >> +                 * LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL or
> > >> +                 * 16M for LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN.
> > >> +                 * Avoid build failures and go with the minimum if less
> > >> +                 * is specified.
> > > 
> > > Build failures? Do you mean "domain build" rather than "libvirt build"?
> > > 
> > > I'm not sure about this fixing up the GIGO from the user side, but
> > > that's a libvirt policy decision I suppose? If it were me I would just
> > > let libxl provide the error and expect people to fix their domain config
> > > rather than silently giving them something other than what they asked
> > > for. What if increasing the VRAM causes a cascading failure due e.g. to
> > > lack of memory? That's going to be tricky to debug I think!
> > 
> > In the end its a start a domain with such a config. Which seems to be what I
> > would end up with in my testing with an admittedly older version of 
> > virt-manager
> > connecting to a libvirtd running an initial version of this patch without 
> > that part.
> > The error seen on the front-end was something along the lines of "failed to 
> > get
> > enough memory to start the guest" (the libxl log on the other side had the
> > better error message). And the gui always reduces the memory below the 
> > minimum
> > for both the options (VGA and "Xen").
> > That is the reason I went for "meh, go for the minimum anyways".
> 
> Does the libvirt protocol require the client to provide all the sizes
> etc with no provision for asking the server side to pick a sane default?

The XML does not have to include the VGA ram size. If it is omitted the
we fill in a default value after initial parsing is done. That defualt
can be hypervisor specific

> 
> > And btw, it is already confusing enough as with cirrus, I get a 9M by 
> > default
> > which is passed on to qemu on the command line and then ignored by it and 
> > one
> > gets 32M in any way...

Yeah, we were silently ignoring ram size for cirrus since QEMU doesn't let
it be configured. Only QXL and I think stdvga is configurable.


Regards,
Daniel
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