On Tue, Jul 21, 2015 at 08:41:33AM -0400, Frediano Ziglio wrote:
On Tue, Jul 21, 2015 at 09:36:55AM +0200, Christophe Fergeau wrote: >On Mon, Jul 20, 2015 at 11:25:52AM +0200, Martin Kletzander wrote: >> I spend all morning fixing this to be installed properly in the >> system. Anyway, I finally managed to make this work and found out the >> guest I used for it is not ready to have multiple monitors. Anyway, >> looking at everything else this definitely works, so ACK, I'll push it >> in a while. > >I'm still a bit worried that all existing guests will have heads='1' in >their XML as libvirt is adding that by default, but I don't really see a >way around it :-/ We should make sure libvirt stops adding it from now >on though ;) > Well, how would you then limit the outputs to 1? Having said that, I have no idea why we started adding heads="1" automatically and playing with different changes, we have another bug in the parsing/formatting code. You'll also won't be able to migrate from older libvirt with heads='1' to newer ones. I haven't realized that. I'm thinking about reverting the change in libvirt or even using heads > 1. Although that won't migrate either. So the only other thing that we can do is to introduce new parameter, like maxHeads. The sound you just heard is me slapping my face because again there will multiple parameters meaning similar things...heads specify the number of heads the emulated virtual card has. I think the problem was specify the number for QXL which never honored this setting. I don't think you can't migrate, just after migration the machine won't allow more than 1 head. Unfortunately the XML file does not have any field to specify the libvirt version was meant for. It would be useful to add so if you see the version it means that heads==1 means 1 if not it means is not defined. I'm against a new parameter which specify the same meaning of another parameter. I would instead something like validHeads or headsSet with a false as default.
[Would you mind convincing your mail client to wrap long lines?] Well, libvirt is, unfortunately, by definition, backward compatible. So we guarantee, that you don't have to change this stuff and you can migrate to newer ones. The problem here with the migration is that the guests will be binary incompatible, the amount of memory you need to transfer from source will not fit the destination. Until we reach a decision, I would revert the patch. We need to decide how to handle this. About the new name. I take it as 'heads' defines how many monitors there are and 'maxHeads' would define the maximum of them to have. The difference would be there of course only for qxl. I don't think cirrus, for example, can dynamically change number of monitors connected, can it? We had similar problem with the ram/vram/vgaram attributes and we ended up with all of them and bunch of lines dealing with just the update and migration compatibility. Having headsSet is pointless. How would you decide whether heads is actually 1 or not if you were to parse an XML with heads='1'? Dropping heads='1' is not an option as well since that would make other video models (that support heads=) change their settings. And nobody could set heads='1' any more without also setting another parameter. The problem is we screwed up when we defaulted heads to a number even though there are devices that do not handle that properly. That's the past, we can't do much about that due to back-compat (Aargh!), but that's how it is, unfortunately). That are my thoughts on why maxHeads should be a new parameter that would set max_outputs for the qxl device.
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