Gerhard Wiesinger wrote: > On Wed, 21 Apr 2010, Jamie Lokier wrote: > > >Gerhard Wiesinger wrote: > >>Hmmm. I'm very new to QEMU and KVM but at least accessing the virtual HW > >>of QEMU even from KVM must be possible (e.g. memory and port accesses are > >>done on nearly every virtual device) and therefore I'm ending in C code in > >>the QEMU hw/*.c directory. Therefore also the VGA memory area should be > >>able to be accessable from KVM but with the specialized and fast memory > >>access of QEMU. Am I missing something? > > > >What you're missing is that when KVM calls out to QEMU to handle > >hw/*.c traps, that call is very slow. It's because the hardware-VM > >support is a bit slow when the trap happens, and then the the call > >from KVM in the kernel up to QEMU is a bit slow again. Then all the > >way back. It adds up to a lot, for every I/O operation. > > Isn't that then a general problem of KVM virtualization (oder hardware > virtualization) in general? Is this CPU dependend (AMD vs. Intel)? Yes it is a general problem, but KVM emulates some time-critical things in the kernel (like APIC and CPU instructions), so it's not too bad. KVM is about 5x faster than TCG for most things, and slower for a few things, so on balance it is usually faster. The slow 256-colour mode writes sound like just a simple bug, though. No need for complicated changes. > >In 256-colour mode, KVM should be writing to the VGA memory at high > >speed a lot like normal RAM, not trapping at the hardware-VM level, > >and not calling up to the code in hw/*.c for every byte. > > Yes, same picture to me: 256 color mode should be only a memory write (16 > color mode is more difficult as pixel/byte mapping is not the same). > But it looks like this isn't the case in this test scenario. > > >You might double-check if your guest is using VGA "Mode X". (See > >Wikipedia.) > > > >That was a way to accelerate VGA on real PCs, but it will be slow in > >KVM for the same reasons as 16-colour mode. > > Which way do you mean? Look up Mode X on Wikipedia if you're interested, but it isn't relevant to the problem you've reported. Mode X cannot be enabled with a BIOS call; it's a VGA hardware programming trick. It would not be useful in a VM environment. -- Jamie -- 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
