linux-kernel - RE: [PATCH 3/3] vfio: Report PASID capability via VFIO_DEVICE

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Message-ID: <BN9PR11MB52765A61C1124F75E8CEFFBC8C8DA@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 13 Dec 2023 02:10:27 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
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        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
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        <shameerali.kolothum.thodi@...wei.com>,
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        "Zeng, Xin" <xin.zeng@...el.com>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: RE: [PATCH 3/3] vfio: Report PASID capability via VFIO_DEVICE_FEATURE
 ioctl

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, December 12, 2023 11:35 PM
> 
> On Mon, Dec 11, 2023 at 11:49:49AM -0700, Alex Williamson wrote:
> > On Mon, 11 Dec 2023 14:10:28 -0400
> > Jason Gunthorpe <jgg@...dia.com> wrote:
> >
> > > On Mon, Dec 11, 2023 at 11:03:45AM -0700, Alex Williamson wrote:
> > > > On Sun, 26 Nov 2023 22:39:09 -0800
> > > > Yi Liu <yi.l.liu@...el.com> wrote:
> >
> > > > >    the PF). Creating a virtual PASID capability in vfio-pci config space
> needs
> > > > >    to find a hole to place it, but doing so may require device specific
> > > > >    knowledge to avoid potential conflict with device specific registers
> like
> > > > >    hiden bits in VF config space. It's simpler by moving this burden to
> the
> > > > >    VMM instead of maintaining a quirk system in the kernel.
> > > >
> > > > This feels a bit like an incomplete solution though and we might
> > > > already posses device specific knowledge in the form of a variant
> > > > driver.  Should this feature structure include a flag + field that
> > > > could serve to generically indicate to the VMM a location for
> > > > implementing the PASID capability?  The default core implementation
> > > > might fill this only for PFs where clearly an emualted PASID capability
> > > > can overlap the physical capability.  Thanks,
> > >
> > > In many ways I would perfer to solve this for good by having a way to
> > > learn a range of available config space - I liked the suggestion to
> > > use a DVSEC to mark empty space.
> >
> > Yes, DVSEC is the most plausible option for the device itself to convey
> > unused config space, but that requires hardware adoption so presumably
> > we're going to need to fill the gaps with device specific code.  That
> > code might live in a variant driver or in the VMM.  If we have faith
> > that DVSEC is the way, it'd make sense for a variant driver to
> > implement a virtual DVSEC to work out the QEMU implementation and set
> a
> > precedent.
> 
> How hard do you think it would be for the kernel to synthesize the
> dvsec if the varient driver can provide a range for it?
> 
> On the other hand I'm not so keen on having variant drivers that are
> only doing this just to avoid a table in qemu :\ It seems like a

me too. If we really want something like this I'd prefer to tracking a
table of device specific ranges instead of requesting full-fledged
variant drivers.