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    SubjectRe: [PATCHv4] uio: add generic driver for PCI 2.3 devices
    On Thu, Jul 16, 2009 at 03:31:01PM +0300, Michael S. Tsirkin wrote:
    > On Wed, Jul 15, 2009 at 03:08:29PM -0700, Greg KH wrote:
    > > On Wed, Jul 15, 2009 at 11:13:40PM +0300, Michael S. Tsirkin wrote:
    > > > This adds a generic uio driver that can bind to any PCI device. First
    > > > user will be virtualization where a qemu userspace process needs to give
    > > > guest OS access to the device.
    > > >
    > > > Interrupts are handled using the Interrupt Disable bit in the PCI command
    > > > register and Interrupt Status bit in the PCI status register. All devices
    > > > compliant to PCI 2.3 (circa 2002) and all compliant PCI Express devices should
    > > > support these bits. Driver detects this support, and won't bind to devices
    > > > which do not support the Interrupt Disable Bit in the command register.
    > > >
    > > > It's expected that more features of interest to virtualization will be
    > > > added to this driver in the future. Possibilities are: mmap for device
    > > > resources, MSI/MSI-X, eventfd (to interface with kvm), iommu.
    > > >
    > > > Acked-by: Chris Wright <chrisw@redhat.com>
    > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    > > > ---
    > > >
    > > > Hans, Greg, please review and consider for upstream.
    > > >
    > > > This is intended to solve the problem in virtualization that shared
    > > > interrupts do not work with assigned devices. Earlier versions of this
    > > > patch have circulated on kvm@vger.
    > >
    > > How does this play with the pci-stub driver that I thought was written
    > > to solve this very problem?
    >
    >
    > AFAIK the problem pci stub was written to solve is simply to bind to a
    > device. You then have to use another kernel module which looks the
    > device up with something like pci_get_bus_and_slot to do anything
    > useful. In particular, for non-shared interrupts, we can disable the
    > interrupt in the apic. But this does not work well for shared
    > interrupts. Thus this work.
    >
    > The uio driver will be used in virtualization scenarious, a couple
    > of possible ones that have been mentioned on the kvm list are:
    > - device assignment (guest access to device) for simple devices with
    > shared interrupts: emulating PCI is tricky enough to better be done in
    > userspace. shared interrupt support is important as it happens
    > with real devices
    > - simple communication between guest and host:
    > we create a virtual device in host, and userspace
    > driver in guest gets events and passes them on
    > to e.g. dbus. shared interrupt support is important
    > to avoid wasting irqs

    Ah, ok, thanks for all of the explanation, that makes sense.

    greg k-h


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