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    SubjectRe: [PATCH v7 2/2] kvm: KVM_EOIFD, an eventfd for EOIs
    On Mon, 2012-08-06 at 13:40 +0300, Avi Kivity wrote:
    > On 08/06/2012 01:38 PM, Avi Kivity wrote:
    > > Regarding the implementation, instead of a linked list, would an array
    > > of counters parallel to the bitmap make it simpler?
    > Or even, replace the bitmap with an array of counters.

    I'm not sure a counter array is what we're really after. That gives us
    reference counting for the irq source IDs, but not the key->gsi lookup.
    It also highlights another issue, that we have a limited set of source
    IDs. Looks like we have BITS_PER_LONG IDs, with two already used, one
    for the shared userspace ID and another for the PIT. How happy are we
    going to be with a limit of 62 level interrupts in use at one time?

    It's arguably a reasonable number since the most virtualization friendly
    devices (sr-iov VFs) don't even support this kind of interrupt. It's
    also very wasteful allocating an entire source ID for a single GSI
    within that source ID. PCI supports interrupts A, B, C, and D, which,
    in the most optimal config, each go to different GSIs. So we could
    theoretically be more efficient in our use and allocation of irq source
    IDs if we tracked use by the source ID, gsi pair.

    That probably makes it less practical to replace anything at the top
    level with a counter array. The key that we pass back is currently the
    actual source ID, but we don't specify what it is, so we could split it
    and have it encode a 16bit source ID plus 16 bit GSI. It could also be
    an idr entry.

    Michael, would the interface be more acceptable to you if we added
    separate ioctls to allocate and free some representation of an irq
    source ID, gsi pair? For instance, an ioctl might return an idr entry
    for an irq source ID/gsi object which would then be passed as a
    parameter in struct kvm_irqfd and struct kvm_eoifd so that the object
    representing the source id/gsi isn't magically freed on it's own. This
    would also allow us to deassign/close one end and reconfigure it later.


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