On 04.06.2011, at 18:49, Sasha Levin wrote: > On Sat, 2011-06-04 at 17:40 +0200, Alexander Graf wrote: >> On 04.06.2011, at 17:34, Sasha Levin wrote: >> >>> On Sat, 2011-06-04 at 17:21 +0200, Alexander Graf wrote: >>>> On 04.06.2011, at 16:19, Sasha Levin wrote: >>>> >>>>> On Sat, 2011-06-04 at 15:48 +0200, Alexander Graf wrote: >>>>>> On 04.06.2011, at 14:04, Sasha Levin wrote: >>>>>> >>>>>>> On Sat, 2011-06-04 at 13:53 +0200, Ingo Molnar wrote: >>>>>>>> * Alexander Graf <[email protected]> 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. >>>>>>> >>>>>>> I doubt we could use dirty pages because unless guest VESA driver >>>>>>> supports panning, it will redraw the entire FB - which means that all >>>>>>> pages will be dirty. >>>>>> >>>>>> Please recheck the math and compare 60 dirty bitmap checks+flushes per >>>>>> second to a few million MMIO exits for every single pixel :). >>>>> >>>>> I might be missing something here, but if every single pixel changes due >>>>> to scrolling, doesn't it mean that all the pages will be marked as >>>>> 'dirty' anyway? >>>> >>>> Sure, but you don't need to exit to user space for every single pixel, but >>>> instead process the whole thing asynchronously. Just run kvm_stat while >>>> running your current implementation and you'll pretty soon realize what >>>> I'm talking about :). >>> >>> I we use coalesced MMIO we only exit when the shared page is full. >> >> Yes, which will be very often for full redrawing guests. Remember, we're >> talking about megabytes of graphics data. Plus you still need to call your >> internal MMIO handler for every single access then. And I hope I don't even >> have to mention read performance (which is abysmal on real graphics cards >> too though). >> >>> If we mark a memory region as log dirty we won't get MMIO exits on it? >> >> [..] If you mark a memory region as coalesced you also don't get MMIO exits >> on it. [..] >> > > We get MMIO exits on it when the ring is full, which is pretty often > with the graphics card. > > I'll try the dirty log method later tonight. I don't see anything about > no exits in the documentation though - if it actually prevents MMIO > exits to the region it should probably be documented.
It's documented. Just look up the documentation for KVM_GET_DIRTY_LOG and KVM_SET_USER_MEMORY_REGION. Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
