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

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 04.04.2014 14:56, Ian Campbell wrote:
> On Fri, 2014-04-04 at 14:51 +0200, Daniel P. Berrange wrote:
>> 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.
> 
> I guess the issue is that whatever client Stefan is using is including
> the VGA ram size, with a value which it turns out is not allowed.

Right and the current fixup code is in there because I am too lazy to be
bothered to use virsh to fix up the vram size all the times. And in some way I
expected other users to be of the same mind. Or even not to find out what went
wrong at all.
I am open to let this drop on the upstream side and only carry the delta
locally. Whichever sounds more suitable for the upstream maintainers.

-Stefan
> 
>> That defualt can be hypervisor specific
> 
> Great!
> 
> Ian.
>>>> 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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