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Date:   Mon, 7 Jun 2021 14:54:54 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Alex Williamson (alex.williamson@redhat.com)" 
        <alex.williamson@redhat.com>, "Raj, Ashok" <ashok.raj@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Robin Murphy <robin.murphy@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        David Gibson <david@gibson.dropbear.id.au>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Jason Wang <jasowang@redhat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal
Message-ID: <20210607175454.GI1002214@nvidia.com>
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On Fri, Jun 04, 2021 at 11:10:53PM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.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.

I thought we concluded this wasn't possible because the guest could
choose to bind the same vPASID to a RID and to a ENQCMD device and
then we run into trouble? Are are you saying that a RID device gets a
complete dedicated table and can always have a vPASID == pPASID?

In any event it needs clear explanation in the next RFC

Jason