linux-kernel - Re: [PATCH v2 0/9] Introduce vfio-pci-core subsystem

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Date:   Thu, 11 Feb 2021 08:36:21 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "liranl@...dia.com" <liranl@...dia.com>,
        "oren@...dia.com" <oren@...dia.com>,
        "tzahio@...dia.com" <tzahio@...dia.com>,
        "leonro@...dia.com" <leonro@...dia.com>,
        "yarong@...dia.com" <yarong@...dia.com>,
        "aviadye@...dia.com" <aviadye@...dia.com>,
        "shahafs@...dia.com" <shahafs@...dia.com>,
        "artemp@...dia.com" <artemp@...dia.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "ACurrid@...dia.com" <ACurrid@...dia.com>,
        "gmataev@...dia.com" <gmataev@...dia.com>,
        "cjia@...dia.com" <cjia@...dia.com>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH v2 0/9] Introduce vfio-pci-core subsystem

On Wed, Feb 10, 2021 at 09:34:52AM -0400, Jason Gunthorpe wrote:
> > I'm a bit confused about the change from v1 to v2, especially about
> > how to inject module specific operations. From live migration p.o.v
> > it may requires two hook points at least for some devices (e.g. i40e 
> > in original Yan's example):
> 
> IMHO, it was too soon to give up on putting the vfio_device_ops in the
> final driver- we should try to define a reasonable public/private
> split of vfio_pci_device as is the norm in the kernel. No reason we
> can't achieve that.
> 
> >  register a migration region and intercept guest writes to specific
> > registers. [PATCH 4/9] demonstrates the former but not the latter
> > (which is allowed in v1).
> 
> And this is why, the ROI to wrapper every vfio op in a PCI op just to
> keep vfio_pci_device completely private is poor :(

Yes.  If Alex has a strong preference to keep some values private
a split between vfio_pci_device vfio_pci_device_priv might be doable,
but it is somewhat silly.

> > Then another question. Once we have this framework in place, do we 
> > mandate this approach for any vendor specific tweak or still allow
> > doing it as vfio_pci_core extensions (such as igd and zdev in this
> > series)?
> 
> I would say no to any further vfio_pci_core extensions that are tied
> to specific PCI devices. Things like zdev are platform features, they
> are not tied to specific PCI devices

Yes, ZDEV is just a special case of exposing extra information for any
PCI device on s390.  It does not fit any split up vfio_pci framework.
In fact I wonder why it even has its own config option.

> > vfio-mdev is just the channel to bring VFIO APIs through mdev core
> > to underlying vendor specific mdev device driver, which is already
> > granted flexibility to tweak whatever needs through mdev_parent_ops.
> 
> This is the second thing, and it could just be deleted. The actual
> final mdev driver can just use vfio_device_ops directly. The
> redirection shim in vfio_mdev.c doesn't add value.

Yes, that would simplify a lot of things.