>But why is it a good thing to do PV drivers in the kernel? You lose >flexibility and functionality to gain performance. Really, it's more >about there not being good enough userspace interfaces to do network IO. > >> The lapic/PIC code >> should also be available in Qemu for OSs that dont have KVM-alike >> support in the kernel. >> >> and while today most of the performance advantages of moving the PIC >> into the kernel are masked by the high cost of VM exits, in the future >> the effect will be more marked, as the relative cost of piggybacking out >> to qemu increases. >> >> I can see the value in doing certain things in Qemu, but i cannot see >> _at all_ the value of handling say the PIT in Qemu. Just look at the >> Qemu PIT/timers code quality in Qemu for a change ... it's a huge ugly >> mess of lots of #ifdefs, ineffective handling of /dev/rtc, linear list >> walking, signal overhead, etc., etc. All of that resulting in 10-15% of >> 'idle' overhead of KVM+qemu when it runs a Linux guest. On the other >> side, in the kernel it's most natural to do timers and to emulate >> hardware, because the kernel has _precise_ knowledge about the >> platform's capabilities.
I think that the pit just need clean, ifdef-less implementation, I can't believe it adds a bug performance penalty. Actually it is quite simple to implement dyn-tick in qemu. The question is after moving the pic,apic,ioapic into the kernel, is it nicer to re-implement the pit in the kernel or in qemu. I suggest we start with the first 3 nominees... >> > >Yeah, I think this is a good point. If we're going to push the APIC >into the kernel, we might as well put the PIT there too. The timing >stuff is an absolute mess in QEMU since it wants to get a fast high res >clock but isn't aware of things like CPU migration. I'm pretty sure >that if you are on an SMP host, some bad things can happen with the QEMU >timer code since it relies on the rdtsc. Btw: KVM support guest rdtsc over physical cpus, I guess it wasn't enough and the qemu pit side need to do it too. > >Regards, > >Anthony Liguori > >> Ingo >> >> > > >----------------------------------------------------------------------- -- >Take Surveys. Earn Cash. Influence the Future of IT >Join SourceForge.net's Techsay panel and you'll get the chance to share >your >opinions on IT & business topics through brief surveys-and earn cash >http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVD EV >_______________________________________________ >kvm-devel mailing list >kvm-devel@lists.sourceforge.net >https://lists.sourceforge.net/lists/listinfo/kvm-devel ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
