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    SubjectRe: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts
    On Mon, Jun 04, 2012 at 01:40:28PM +0200, Jan Kiszka wrote:
    > On 2012-06-04 13:21, Thomas Gleixner wrote:
    > > On Sun, 3 Jun 2012, Avi Kivity wrote:
    > >
    > >> On 06/01/2012 09:26 PM, Jan Kiszka wrote:
    > >>>
    > >>>> you suggesting we need a request_edge_threaded_only_irq() API? Thanks,
    > >>>
    > >>> I'm just wondering if that restriction for threaded IRQs is really
    > >>> necessary for all use cases we have. Threaded MSIs do not appear to me
    > >>> like have to be handled that conservatively, but maybe I'm missing some
    > >>> detail.
    > >>>
    > >>
    > >> btw, I'm hoping we can unthread assigned MSIs. If the delivery is
    > >> unicast, we can precalculate everything and all the handler has to do is
    > >> set the IRR, KVM_REQ_EVENT, and kick the vcpu. All of these can be done
    > >> from interrupt context with just RCU locking.
    > >
    > > There is really no need to run MSI/MSI-X interrupts threaded for
    > > KVM. I'm running the patch below for quite some time and it works like
    > > a charm.
    > >
    > > Thanks,
    > >
    > > tglx
    > > ----


    ....

    >
    > This may work in practice but has two conceptual problems:
    > - we do not want to run a potential broadcast to all VCPUs to run in
    > a host IRQ handler

    I'm not sure why this one is a problem: injecting an interrupt
    once you know the vcpu seems really cheap.
    It's true that scanning vcpus might take a bit more time
    when there are lots of them but it's a single
    linear scan that we do anyway.

    And we also inject msi from irqfd callback with interrupts
    disabled which seems equivalent.

    Pls correct me if I'm wrong.

    > - crazy user space could have configured the route to end up in the
    > PIC or IOAPIC, and both are not hard-IRQ safe (this should probably
    > be caught on setup)

    Yes this needs to be fixed up.

    > So this shortcut requires some checks before being applied to a specific
    > MSI/MSI-X vector.
    >
    >
    > Taking KVM aside, my general question remains if threaded MSI handlers
    > of all devices really need to apply IRQF_ONESHOT though they should have
    > no use for it.
    >
    > Jan
    >
    > --
    > Siemens AG, Corporate Technology, CT T DE IT 1
    > Corporate Competence Center Embedded Linux
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