[Xen-devel] [rfc 0/2] allow pass-through devices to share GSI

[Simon Horman <horms@xxxxxxxxxxxx>]

As discussed recently in the thread "[rfc 00/18] ioemu: use devfn instead
of slots as the unit for passthrough", it would be useful if pass-through
devices could share GSI.

The motivation for this is:

* Allow multi-function devices to be passed through as multi-function devices
  - This implies that the devices may have functions that use INTB, C or D.
    With the current scheme this would clash with the GSI of INTA on
    device 13, 14, 15, 21, 22, 23, 29 ,30 and 31.

    The INTX, device to GSI mapping is described in
    xen/include/asm-x86/hvm/irq.h as:

    #define hvm_pci_intx_gsi(dev, intx)  \
        (((((dev)<<2) + ((dev)>>3) + (intx)) & 31) + 16)

    And is illustrated in this diagram
    http://lists.xensource.com/archives/html/xen-devel/2009-02/pngmIO1Sm0VEX.png

* Allow more than two pass-through devices.
  - This will place more contention on the GSI-space, and allocation
    becomes a lot simpler if GSI sharing is allowed.

-- 
Simon Horman
  VA Linux Systems Japan K.K., Sydney, Australia Satellite Office
  H: www.vergenet.net/~horms/             W: www.valinux.co.jp/en


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