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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Alex Williamson <alex.williamson@redhat.com>
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Subject: RE: [RFC PATCH V3 00/26] vfio/pci: Back guest interrupts from
 Interrupt Message Store (IMS)
Thread-Topic: [RFC PATCH V3 00/26] vfio/pci: Back guest interrupts from
 Interrupt Message Store (IMS)
Thread-Index: AQHaCPdEtaV8qeHO6EmMpiKLQ58cQ7BjhMFwgAJFzACAAIkGYIAADu6Q
Date:   Thu, 2 Nov 2023 03:14:09 +0000
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> From: Tian, Kevin
> Sent: Thursday, November 2, 2023 10:52 AM
>=20
> >
> > Without an in-tree user of this code, we're just chopping up code for
> > no real purpose.  There's no reason that a variant driver requiring IMS
> > couldn't initially implement their own SET_IRQS ioctl.  Doing that
>=20
> this is an interesting idea. We haven't seen a real usage which wants
> such MSI emulation on IMS for variant drivers. but if the code is
> simple enough to demonstrate the 1st user of IMS it might not be
> a bad choice. There are additional trap-emulation required in the
> device MMIO bar (mostly copying MSI permission entry which contains
> PASID info to the corresponding IMS entry). At a glance that area
> is 4k-aligned so should be doable.
>=20

misread the spec. the MSI-X permission table which provides
auxiliary data to MSI-X table is not 4k-aligned. It sits in the 1st
4k page together with many other registers. emulation of them
could be simple with a native read/write handler but not sure
whether any of them may sit in a hot path to affect perf due to
trap...