From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752425AbbJFOi2 (ORCPT ); Tue, 6 Oct 2015 10:38:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45869 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751498AbbJFOi1 (ORCPT ); Tue, 6 Oct 2015 10:38:27 -0400 Date: Tue, 6 Oct 2015 17:38:21 +0300 From: "Michael S. Tsirkin" To: Avi Kivity Cc: Greg KH , Vlad Zolotarov , linux-kernel@vger.kernel.org, hjk@hansjkoch.de, corbet@lwn.net, bruce.richardson@intel.com, avi@cloudius-systems.com, gleb@cloudius-systems.com, stephen@networkplumber.org, alexander.duyck@gmail.com Subject: Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support Message-ID: <20151006143821.GA11541@redhat.com> References: <1443991398-23761-1-git-send-email-vladz@cloudius-systems.com> <1443991398-23761-3-git-send-email-vladz@cloudius-systems.com> <20151005031159.GB27303@kroah.com> <56123493.9000602@scylladb.com> <20151005094932.GA5236@kroah.com> <56124EDB.3070701@scylladb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56124EDB.3070701@scylladb.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 05, 2015 at 01:20:11PM +0300, Avi Kivity wrote: > On 10/05/2015 12:49 PM, Greg KH wrote: > >On Mon, Oct 05, 2015 at 11:28:03AM +0300, Avi Kivity wrote: > >>Of course it has to be documented, but this just follows vfio. > >> > >>Eventfd is a natural enough representation of an interrupt; both kvm and > >>vfio use it, and are also able to share the eventfd, allowing a vfio > >>interrupt to generate a kvm interrupt, without userspace intervention, and > >>one day without even kernel intervention. > >That's nice and wonderful, but it's not how UIO works today, so this is > >now going to be a mix and match type interface, with no justification so > >far as to why to create this new api and exactly how this is all going > >to be used from userspace. > > The intended user is dpdk (http://dpdk.org), which is a family of userspace > networking drivers for high performance networking applications. > > The natural device driver for dpdk is vfio, which both provides memory > protection and exposes msi/msix interrupts. However, in many cases vfio > cannot be used, either due to the lack of an iommu (for example, in > virtualized environments) or out of a desire to avoid the iommus performance > impact. > > The challenge in exposing msix interrupts to user space is that there are > many of them, so you can't simply poll the device fd. If you do, how do you > know which interrupt was triggered? The solution that vfio adopted was to > associate each interrupt with an eventfd, allowing it to be individually > polled. Since you can pass an eventfd with SCM_RIGHTS, and since kvm can > trigger guest interrupts using an eventfd, the solution is very flexible. > > >Example code would be even better... > > > > > > > This is the vfio dpdk interface code: > > http://dpdk.org/browse/dpdk/tree/lib/librte_eal/linuxapp/eal/eal_pci_vfio.c > > basically, the equivalent uio msix code would be very similar if uio adopts > a similar interface: > > http://dpdk.org/browse/dpdk/tree/lib/librte_eal/linuxapp/eal/eal_pci_uio.c > > (current code lacks msi/msix support, of course). So you really want a driver that behaves exactly like vfio. Which immediately begs a question: why not extend vfio to cover your usecase. -- MST