RE: [PATCH v3 03/10] iommu/sva: Manage process address spaces

["Liu, Yi L" <yi.l.liu@intel.com>]

> From: iommu-bounces@lists.linux-foundation.org [mailto:iommu-
> bounces@lists.linux-foundation.org] On Behalf Of Jean-Philippe Brucker
> Sent: Wednesday, September 26, 2018 9:50 PM
> Subject: Re: [PATCH v3 03/10] iommu/sva: Manage process address spaces
> 
> On 26/09/2018 13:45, Joerg Roedel wrote:
> > On Wed, Sep 26, 2018 at 11:20:34AM +0100, Jean-Philippe Brucker wrote:
> >> Yes, at the moment it's difficult to guess what device drivers will
> >> want, but I can imagine some driver offering SVA to userspace, while
> >> keeping a few PASIDs for themselves to map kernel memory. Or create mdev
> >> devices for virtualization while also allowing bare-metal SVA. So I
> >> think we should aim at enabling these use-cases in parallel, even if it
> >> doesn't necessarily need to be possible right now.
> >
> > Yeah okay, but allowing these use-cases in parallel basically disallows
> > giving any guest control over a device's pasid-table, no?
> All of these use-cases require the host to manage the PASID tables, so
> while any one of them is enabled, we can't give a guest control over the
> PASID tables. But allowing these use-cases in parallel doesn't change that.
> 
> There is an ambiguity: I understand "(3) SVA in VM guest" as SVA for a
> device-assignable interface assigned to a guest, using vfio-mdev and the
> new Intel vt-d architecture (right?). That case does require the host to
> allocate and manage PASIDs (because the PCI device is shared between
> multiple VMs).

Correct. For such case, we give host the charge to allocate and manage PASIDs.
And the reason is correct.

> 
> For the "classic" vfio-pci case, "SVA in guest" still means giving the
> guest control over the whole PASID table.

No, if giving guest control over the whole PASID table, it means guest may have
its own PASID namespace. right? And for vfio-mdev case, it gets PASID from host.
So there would be multiple PASID namespaces. Thinking about the following scenario:

A PF/VF assigned to a VM via "classic" vfio-pci. And an assignable-device-interface
assigned to this VM via vfio-mdev. If an application in this VM tries to manipulate
these two "devices", it should have the same PASID programmed to them. right?
But as the above comments mentioned, for vfio-pci case, it would get a PASID from
its own PASID namespace. While the vfio-mdev case would get a PASID from host.
This would result in conflict.

So I would like we get host to allocate and manage the whole PASID table, so that
to cover possible combinations of vfio-pci passthru and vfio-mdev passthru.

> 
> Thanks,
> Jean

Regards,
Yi Liu