Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758206Ab2F1Dw5 (ORCPT ); Wed, 27 Jun 2012 23:52:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38588 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757941Ab2F1Dw4 (ORCPT ); Wed, 27 Jun 2012 23:52:56 -0400 Message-ID: <1340855572.1207.300.camel@bling.home> Subject: Re: [PATCH v2 4/6] kvm: Extend irqfd to support level interrupts From: Alex Williamson To: "Michael S. Tsirkin" Cc: avi@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, jan.kiszka@siemens.com Date: Wed, 27 Jun 2012 21:52:52 -0600 In-Reply-To: <20120627222852.GA27885@redhat.com> References: <20120627044758.23698.249.stgit@bling.home> <20120627050937.23698.68442.stgit@bling.home> <20120627095137.GH17507@redhat.com> <1340830749.1207.236.camel@bling.home> <20120627211413.GE27746@redhat.com> <1340832499.1207.255.camel@bling.home> <20120627222852.GA27885@redhat.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4981 Lines: 92 On Thu, 2012-06-28 at 01:28 +0300, Michael S. Tsirkin wrote: > On Wed, Jun 27, 2012 at 03:28:19PM -0600, Alex Williamson wrote: > > On Thu, 2012-06-28 at 00:14 +0300, Michael S. Tsirkin wrote: > > > On Wed, Jun 27, 2012 at 02:59:09PM -0600, Alex Williamson wrote: > > > > On Wed, 2012-06-27 at 12:51 +0300, Michael S. Tsirkin wrote: > > > > > On Tue, Jun 26, 2012 at 11:09:46PM -0600, Alex Williamson wrote: > > > > > > In order to inject an interrupt from an external source using an > > > > > > irqfd, we need to allocate a new irq_source_id. This allows us to > > > > > > assert and (later) de-assert an interrupt line independently from > > > > > > users of KVM_IRQ_LINE and avoid lost interrupts. > > > > > > > > > > > > We also add what may appear like a bit of excessive infrastructure > > > > > > around an object for storing this irq_source_id. However, notice > > > > > > that we only provide a way to assert the interrupt here. A follow-on > > > > > > interface will make use of the same irq_source_id to allow de-assert. > > > > > > > > > > > > Signed-off-by: Alex Williamson > > > > > > --- > > > > > > > > > > > > Documentation/virtual/kvm/api.txt | 5 ++ > > > > > > arch/x86/kvm/x86.c | 1 > > > > > > include/linux/kvm.h | 3 + > > > > > > virt/kvm/eventfd.c | 95 +++++++++++++++++++++++++++++++++++-- > > > > > > 4 files changed, 99 insertions(+), 5 deletions(-) > > > > > > > > > > > > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt > > > > > > index ea9edce..b216709 100644 > > > > > > --- a/Documentation/virtual/kvm/api.txt > > > > > > +++ b/Documentation/virtual/kvm/api.txt > > > > > > @@ -1981,6 +1981,11 @@ the guest using the specified gsi pin. The irqfd is removed using > > > > > > the KVM_IRQFD_FLAG_DEASSIGN flag, specifying both kvm_irqfd.fd > > > > > > and kvm_irqfd.gsi. > > > > > > > > > > > > +With KVM_IRQFD_FLAG_LEVEL KVM_IRQFD allocates a new IRQ source ID for > > > > > > +the requested irqfd. This is necessary to share level triggered > > > > > > +interrupts with those injected through KVM_IRQ_LINE. IRQFDs created > > > > > > +with KVM_IRQFD_FLAG_LEVEL must also set this flag when de-assiging. > > > > > > +KVM_IRQFD_FLAG_LEVEL support is indicated by KVM_CAP_IRQFD_LEVEL. > > > > > > > > > > Note that if my patch removing auto-deassert gets accepted, > > > > > this is not needed at all: we can just look at the GSI > > > > > to see if it's level or edge. > > > > > > > > I'm not sure this is a good idea. I know from vfio that I'm injecting a > > > > level interrupt regardless of how the guest has the pic/ioapic > > > > programmed at the time I'm calling this ioctl. Peeking across address > > > > spaces to get to the right pin on the right pic/ioapic and see how it's > > > > currently programmed seems fragile. Thanks, > > > > > > > > Alex > > > > > > Fragile? If you set eventfd as LEVEL but GSI is really edge then > > > it all explodes, right? So why give users the option to shoot > > > themselves in the foot? > > > > If the guest has the ioapic rte set to edge at the time I call KVM_IRQFD > > to register my level interrupt then it all explodes, right? I'd rather > > let the user shoot themselves than play Russian roulette with the guest. > > Am I misunderstanding what you mean by looking that the GSI to see if > > it's level or edge? > > Not sure. > I simply mean this: if eventfd is bound to irqfd, set level from irqfd > and clear from eventfd ack notifier. Are you simply saying assert (kvm_set_irq(,,,1)) from irqfd trigger and de-assert (kvm_set_irq(,,,0)) from eventfd ack notifier (aka KVM_EOIFD)? > There's no need for a special IRQ_LEVEL for this. That ignores the whole problem of when do we need to allocate a new irq_source_id and when do we inject using KVM_USERSPACE_IRQ_SOURCE_ID. We've already discussed that a level triggered, externally fired interrupt must use a separate source ID from Qemu userspace. Therefore when you say "look at the GSI to see if it's level or edge", I assume you mean trace the gsi back to the pic/ioapic pin and look at the trigger mode. That trigger mode is configured by the guest, so that means that at the point in time when we call KVM_IRQFD we make a determination based on how the _guest_ has programmed the ioapic. That may not match the interrupt we expect to inject. On the other hand, the user calling KVM_IRQFD absolutely knows the type of interrupt provided by their device. I think we need a flag regardless of whether your patch is accepted. We may be able to share the inject handler if it is accepted, but it doesn't change the user API. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/