> -----Original Message----- > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx] > Sent: Saturday, June 13, 2015 3:04 AM > To: Avi Kivity > Cc: Wu, Feng; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > pbonzini@xxxxxxxxxx; mtosatti@xxxxxxxxxx; eric.auger@xxxxxxxxxx > Subject: Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding > > On Fri, 2015-06-12 at 21:48 +0300, Avi Kivity wrote: > > On 06/12/2015 06:41 PM, Alex Williamson wrote: > > > On Fri, 2015-06-12 at 00:23 +0000, Wu, Feng wrote: > > >>> -----Original Message----- > > >>> From: Avi Kivity [mailto:avi.kivity@xxxxxxxxx] > > >>> Sent: Friday, June 12, 2015 3:59 AM > > >>> To: Wu, Feng; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx > > >>> Cc: pbonzini@xxxxxxxxxx; mtosatti@xxxxxxxxxx; > > >>> alex.williamson@xxxxxxxxxx; eric.auger@xxxxxxxxxx > > >>> Subject: Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding > > >>> > > >>> On 06/11/2015 01:51 PM, Feng Wu wrote: > > >>>> From: Eric Auger <eric.auger@xxxxxxxxxx> > > >>>> > > >>>> This patch adds and documents a new KVM_DEV_VFIO_DEVICE group > > >>>> and 2 device attributes: KVM_DEV_VFIO_DEVICE_FORWARD_IRQ, > > >>>> KVM_DEV_VFIO_DEVICE_UNFORWARD_IRQ. The purpose is to be able > > >>>> to set a VFIO device IRQ as forwarded or not forwarded. > > >>>> the command takes as argument a handle to a new struct named > > >>>> kvm_vfio_dev_irq. > > >>> Is there no way to do this automatically? After all, vfio knows that a > > >>> device interrupt is forwarded to some eventfd, and kvm knows that some > > >>> eventfd is forwarded to a guest interrupt. If they compare notes > > >>> through a central registry, they can figure out that the interrupt needs > > >>> to be forwarded. > > >> Oh, just like Eric mentioned in his reply, this description is out of context of > > >> this series, I will remove them in the next version. > > > > > > I suspect Avi's question was more general. While forward/unforward is > > > out of context for this series, it's very similar in nature to > > > enabling/disabling posted interrupts. So I think the question remains > > > whether we really need userspace to participate in creating this > > > shortcut or if kvm and vfio can some how orchestrate figuring it out > > > automatically. > > > > > > Personally I don't know how we could do it automatically. We've always > > > relied on userspace to independently setup vfio and kvm such that > > > neither have any idea that the other is there and update each side > > > independently when anything changes. So it seems consistent to continue > > > that here. It doesn't seem like there's much to gain performance-wise > > > either, updates should be a relatively rare event I'd expect. > > > > > > There's really no metadata associated with an eventfd, so "comparing > > > notes" automatically might imply some central registration entity. That > > > immediately sounds like a much more complex solution, but maybe Avi has > > > some ideas to manage it. Thanks, > > > > > > > The idea is to have a central registry maintained by a posted interrupts > > manager. Both vfio and kvm pass the filp (along with extra information) > > to the posted interrupts manager, which, when it detects a filp match, > > tells each of them what to do. > > > > The advantages are: > > - old userspace gains the optimization without change > > - a userspace API is more expensive to maintain than internal kernel > > interfaces (CVEs, documentation, maintaining backwards compatibility) > > - if you can do it without a new interface, this indicates that all the > > information in the new interface is redundant. That means you have to > > check it for consistency with the existing information, so it's extra > > work (likely, it's exactly what the posted interrupt manager would be > > doing anyway). > > Yep, those all sound like good things and I believe that's similar in > design to the way we had originally discussed this interaction at > LPC/KVM Forum several years ago. I'd be in favor of that approach. > Thanks, This seems a little complex compared to the current solution, since I am not quite familiar with VFIO, Alex, can you help on this if we need to do this that way, especially for the VFIO part? Thanks, Feng > > Alex ��.n��������+%������w��{.n�����o�^n�r������&��z�ޗ�zf���h���~����������_��+v���)ߣ�
