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

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Message-ID: <BN9PR11MB52768A15A295455C9D82AA8B8CA8A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 8 Nov 2023 09:16:14 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     "Chatre, Reinette" <reinette.chatre@...el.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: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, November 8, 2023 10:50 AM
> 
> On Tue, Nov 07, 2023 at 04:06:41PM -0700, Alex Williamson wrote:
> 
> > This driver that doesn't exist yet could implement its own SET_IRQS
> > ioctl that backs MSI-X with IMS as a starting point.  Presumably we
> > expect multiple drivers to require this behavior, so common code makes
> > sense, but the rest of us in the community can't really evaluate how
> > much it makes sense to slice the common code without seeing that
> > implementation and how it might leverage, if not directly use, the
> > existing core code.
> 
> I've been seeing a general interest in taking something that is not
> MSI-X (eg "IMS" for IDXD) and converting it into MSI-X for the vPCI
> function. I think this will be a durable need in this space.
> 
> Ideally it will be overtaken by simply teaching the guest, vfio and
> the hypervisor interrupt logic how to directly generate interrupts
> with a guest controlled addr/data pair without requiring MSI-X
> trapping. That is the fundamental reason why this has to be done this
> convoluted way.
> 

Even with that a legacy guest which doesn't support such enlightened
way still needs this convoluted way. 😊

and for SIOV anyway the trap cannot be eliminated given the interrupt
storage is shared by all vdev's.