linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Fri, 4 Jun 2021 09:09:03 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Fri, Jun 04, 2021 at 06:37:26AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe
> > Sent: Thursday, June 3, 2021 9:05 PM
> > 
> > > >
> > > > 3) Device accepts any PASIDs from the guest. No
> > > >    vPASID/pPASID translation is possible. (classic vfio_pci)
> > > > 4) Device accepts any PASID from the guest and has an
> > > >    internal vPASID/pPASID translation (enhanced vfio_pci)
> > >
> > > what is enhanced vfio_pci? In my writing this is for mdev
> > > which doesn't support ENQCMD
> > 
> > This is a vfio_pci that mediates some element of the device interface
> > to communicate the vPASID/pPASID table to the device, using Max's
> > series for vfio_pci drivers to inject itself into VFIO.
> > 
> > For instance a device might send a message through the PF that the VF
> > has a certain vPASID/pPASID translation table. This would be useful
> > for devices that cannot use ENQCMD but still want to support migration
> > and thus need vPASID.
> 
> I still don't quite get. If it's a PCI device why is PASID translation required?
> Just delegate the per-RID PASID space to user as type-3 then migrating the 
> vPASID space is just straightforward.

This is only possible if we get rid of the global pPASID allocation
(honestly is my preference as it makes the HW a lot simpler)

Without vPASID the migration would need pPASID's on the RID that are
guarenteed free.

Jason