On Thu, Mar 18, 2010 at 11:11:43AM -0500, Anthony Liguori wrote: > On 03/18/2010 10:17 AM, Ingo Molnar wrote: > >* Anthony Liguori<anthony@xxxxxxxxxxxxx> wrote: > >>On 03/18/2010 08:00 AM, Ingo Molnar wrote: > >>>>[...] kvm in fact knows nothing about vga, to take your last example. > >>>>[...] > >>>> > >>>Look at the VGA dirty bitmap optimization a'ka the KVM_GET_DIRTY_LOG > >>>ioctl. > >>> > >>>See qemu/kvm-all.c's kvm_physical_sync_dirty_bitmap(). > >>> > >>>It started out as a VGA optimization (also used by live migration) and > >>>even today it's mostly used by the VGA drivers - albeit a weak one. > >>> > >>>I wish there were stronger VGA optimizations implemented, copying the > >>>dirty bitmap is not a particularly performant solution. (although it's > >>>certainly better than full emulation) Graphics performance is one of the > >>>more painful aspects of KVM usability today. > >>> > >>We have to maintain a dirty bitmap because we don't have a paravirtual > >>graphics driver. IOW, someone needs to write an Xorg driver. > >> > >>Ideally, we could just implement a Linux framebuffer device, right? > >> > >No, you'd want to interact with DRM. > > Using DRM doesn't help very much. You still need an X driver and most > of the operations you care about (video rendering, window movement, etc) > are not operations that need to go through DRM. > > 3D graphics virtualization is extremely difficult in the non-passthrough > case. It really requires hardware support that isn't widely available > today (outside a few NVIDIA chipsets). > Implementing a virtualized DRM/KMS driver would at least get you the framebuffer interface more or less for free, while allowing you to deal with the userspace side of things incrementally (ie, running a dummy xorg on top of the virtualized fbdev until the DRI side catches up). It would also enable you to focus on the 2D and 3D parts independently. > It doesn't provide the things we need to a good user experience. You > need things like an absolute input device, host driven display resize, > RGBA hardware cursors. None of these go through DRI and it's those > things that really provide the graphics user experience. > None of these things negate the benefit one would get from a virtualized DRM/KMS driver either. There are multiple problems that need solving in this area, and it's a bit disingenuous to discount a valid suggestion out of hand due to the fact it doesn't solve all of the outstanding issues. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
