Re: [PATCH] kvm tools, ui: Optimize SDL updates

[Ingo Molnar <mingo@elte.hu>]

* Alexander Graf <agraf@suse.de> wrote:

> Why would you need panning/scrolling for a fast FB? It's really an 
> optimization that helps a lot with VNC, but on local machines or 
> SDL you shouldn't see a major difference.

Qemu's fb console scrolling graphics is pretty slow to me even 
locally so i assume that the dirty bitmap trick is not enough.

VirtualBox graphics is very fast, but it probably has its own console 
abstraction and scrolling/2D/3D acceleration.

Also, since tools/kvm/ is really also about learning interesting 
stuff, smooth scrolling was the historic first 'acceleration' usecase 
that video graphics cards added - before they evolved more complex 2D 
acceleration and then started doing 3D.

Walking that path would allow us to do a gradual approach, while 
still having relevant functionality and enhancements at every step.

> Unless you use the FB as MMIO. Qemu just maps the FB as RAM and 
> checks for dirty bitmap updates periodically. That way you don't 
> constantly exit due to MMIO and are good on speed. The slowness you 
> describe sounds a lot as if you don't do that trick.

Correct, and i assumed we already do the dirty-bitmap trick:

	KVM_MEM_LOG_DIRTY_PAGES
	KVM_GET_DIRTY_LOG

But you are right, we do not actually do that!

Pekka, i think this should be the next step. We'll need scrolling 
after that ...

In theory it would also be nice to tunnel the VGA text frame buffer 
over to the KVM tool - as serial console is not supported by most 
installers and default distro images. We could actually do a rather 
good job of emulating it via Slang/Curses.

Thanks,

	Ingo