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

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Date:   Thu, 2 Nov 2023 02:51:40 +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: Alex Williamson <alex.williamson@...hat.com>
> Sent: Thursday, November 2, 2023 2:07 AM
> 
> On Tue, 31 Oct 2023 07:31:24 +0000
> "Tian, Kevin" <kevin.tian@...el.com> wrote:
> 
> > > From: Chatre, Reinette <reinette.chatre@...el.com>
> > > Sent: Saturday, October 28, 2023 1:01 AM
> > >
> > > Changes since RFC V2:
> > > - RFC V2:
> > >
> https://lore.kernel.org/lkml/cover.1696609476.git.reinette.chatre@intel.com
> > > /
> > > - Still submiting this as RFC series. I believe that this now matches the
> > >   expectatations raised during earlier reviews. If you agree this is
> > >   the right direction then I can drop the RFC prefix on next submission.
> > >   If you do not agree then please do let me know where I missed
> > >   expectations.
> >
> > Overall this matches my expectation. Let's wait for Alex/Jason's thoughts
> > before moving to next-level refinement.
> 
> It feels like there's a lot of gratuitous change without any clear
> purpose.  We create an ops structure so that a variant/mdev driver can
> make use of the vfio-pci-core set_irqs ioctl piecemeal, but then the
> two entry points that are actually implemented by the ims version are
> the same as the core version, so the ops appear to be at the wrong
> level.  The use of the priv pointer for the core callbacks looks like
> it's just trying to justify the existence of the opaque pointer, it
> should really just be using container_of().  We drill down into various
> support functions for MSI (ie. enable, disable, request_interrupt,
> free_interrupt, device name), but INTx is largely ignored, where we
> haven't even kept is_intx() consistent with the other helpers.

All above are good points. The main interest of this series is to share
MSI frontend interface with various backends (PCI MSI/X, IMS, and
purely emulated). From this angle the current ops abstraction does
sound to sit in a wrong level. But if counting your suggestion to also
refactor mdev sample driver (e.g. mtty emulates INTx) then there
might be a different outcome.

> 
> 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.

let's explore more into this option.

> might lead to a more organic solution where we create interfaces where
> they're actually needed.  The existing mdev sample drivers should also
> be considered in any schemes to refactor the core code into a generic
> SET_IRQS helper for devices exposing a vfio-pci API.  Thanks,
> 

In this case we'll have mtty to demonstrate an emulated INTx backend
and intel vgpu to contain an emulated MSI backend.

and moving forward all drivers with a vfio-pci API should share a same
frontend interface.