Arjan van de Ven wrote: >>> shared interrupts aren't a big deal in Linux. at all. >>> In fact, sharing all PV interrupts to one number is a performance >>> enhancement ;) >>> >>> >> If you share a PV interrupt with a non-PV interrupt, then for each PV >> interrupt you have to check whether the non-PV interrupt fired. That >> involves at least one expensive vmexit. >> > > hmm not really. > the completion ring lives in guest memory, the guest can check it > without vmexit entirely. > The PV block driver only does 2 vm events normally (apart from boot time > setup), one to inform the host new IO has been submitted in the submit > ring, and one to receive an interrupt to get notified one or more > completions are present in the completion ring. Nothing more.... > >
But doesn't the interrupt code have to check all interrupt sources on a shared line? That means checking the non-PV interrupt as well, which involves a vmexit (for mmio read). >> I agree that sharing PV interrupts is not very expensive (though I don't >> see why you call it an optimization - if you share 100 interrupts on one >> line you need to check 100 interrupt sources every time the interrupt >> fires). >> > > as long as the check is cheap that is actually fine; if in your 100 > sources scenario 10 have fired, you only get 1 vm event to deliver the > irq, and then 100 polls of guest memory inside the guest. That should be > cheaper than doing 10 real vm interrupts. > It is possible (Xen does it) to batch interrupt delivery so that multiple interrupts can be delivered with one vmentry. So you get the best of both worlds: one check for interrupt source, and one vmentry if multiple interrupts have fired. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. ------------------------------------------------------------------------- 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 [email protected] https://lists.sourceforge.net/lists/listinfo/kvm-devel
