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

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Date:   Tue, 7 Nov 2023 22:49:53 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Reinette Chatre <reinette.chatre@...el.com>,
        "Tian, Kevin" <kevin.tian@...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)

On Tue, Nov 07, 2023 at 04:06:41PM -0700, Alex Williamson wrote:

> A vfio-pci variant driver is specifically a driver that leverages
> portions of vfio-pci-core for implementing vfio_device_ops and binds to
> a PCI device.  It might actually be the wrong term here, but I jumped
> to that since the series tries to generalize portions of one of the
> vfio-pci-core code paths. You might very well be intending to use this
> with something more like an mdev driver, which is fine.

IDXD will be a SIOV device and we need to have a serious talk about
how SIOV device lifecycle will work..

> That also sort of illustrates the point though that this series is
> taking a pretty broad approach to slicing up vfio-pci-core's SET_IRQS
> ioctl code path, enabling support for IMS backed interrupts, but in
> effect complicating the whole thing without any actual consumer to
> justify the complication.  Meanwhile I think the goal is to reduce
> complication to a driver that doesn't exist yet.  So it currently seems
> like a poor trade-off.

I think we need to see some draft of the IDXD driver to really
understand this

> 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.

Jason