linux-kernel - RE: [RFC PATCH V3 00/26] vfio/pci: Back guest interrupts from Interrupt Message Store (IMS)

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Message-ID: <BN9PR11MB52769292F138F69D8717BE8D8CA6A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 2 Nov 2023 03:14:09 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "Chatre, Reinette" <reinette.chatre@...el.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "tom.zanussi@...ux.intel.com" <tom.zanussi@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: RE: [RFC PATCH V3 00/26] vfio/pci: Back guest interrupts from
 Interrupt Message Store (IMS)

> From: Tian, Kevin
> Sent: Thursday, November 2, 2023 10:52 AM
> 
> >
> > 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
> 
> 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.
> 

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...