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

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Message-ID: <MWHPR11MB18863B2D785138E12193983A8C3B9@MWHPR11MB1886.namprd11.prod.outlook.com>
Date:   Fri, 4 Jun 2021 23:10:53 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.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

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Friday, June 4, 2021 8:09 PM
> 
> 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)
> 

In this proposal global vs. per-RID allocation is a per-device policy.
for vfio-pci it can always use per-RID (regardless of whether the
device is partially mediated or not) and no vPASID/pPASID conversion. 
Even for mdev if no ENQCMD we can still do per-RID conversion.
only for mdev which has ENQCMD we need global pPASID allocation.

I think this is the motivation you explained earlier that it's not good
to have one global PASID allocator in the kernel. per-RID vs. global
should be selected per device.

Thanks
Kevin