linux-kernel - Re: vPASID capability for VF

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Message-ID: <ZFrThMhUnsYOE3WP@nvidia.com>
Date:   Tue, 9 May 2023 20:13:08 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        "Lu, Baolu" <baolu.lu@...el.com>
Subject: Re: vPASID capability for VF

On Tue, May 09, 2023 at 10:57:04PM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Wednesday, May 10, 2023 6:44 AM
> > 
> > On Tue, May 09, 2023 at 08:34:53AM +0000, Tian, Kevin wrote:
> > > According to PCIe spec (7.8.9 PASID Extended Capability Structure):
> > >
> > >   The PASID configuration of the single non-VF Function representing
> > >   the device is also used by all VFs in the device. A PF is permitted
> > >   to implement the PASID capability, but VFs must not implement it.
> > >
> > > To enable PASID on VF then one open is where to locate the PASID
> > > capability in VF's vconfig space. vfio-pci doesn't know which offset
> > > may contain VF specific config registers. Finding such offset must
> > > come from a device specific knowledge.
> > 
> > Why? Can't vfio probe the cap tree and just find a gap to insert a new
> > cap? We already mangle the cap list, I'm not sure I see what
> > the problem is?
> > 
> 
> PCI config space includes not only caps, but also device specific
> defined fields. e.g. Intel IGD defines offset 0xfc as a pointer to
> OpRegion. I'm sure Alex can give many other examples.

Do we even expose those over VIFO? I thought in general we blocked of
various parts of the config space. I keep seeing patches to unblock
parts of config space?

I'd do the reverse and say devices that want to pass parts of their
config space should have a special hook to do it and otherwise we
should sanitize and block?

eg we already have a hook to pass the opregion

> So it's easy to find the gap between caps, but not easy to know
> whether that gap is actually free to use.

Because, let's face it, this is a horrible thing to do, and the
opregion stuff is just ugly as s sin.

Jason