Re: [PATCH 0/9 v2] vfio-pci: add support for Freescale IOMMU (PAMU)

[Scott Wood <scottwood@freescale.com>]

On Thu, 2013-11-21 at 13:43 -0700, Alex Williamson wrote:
> On Thu, 2013-11-21 at 11:20 +0000, Bharat Bhushan wrote:
> > 
> > > -----Original Message-----
> > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > Sent: Thursday, November 21, 2013 12:17 AM
> > > To: Bhushan Bharat-R65777
> > > Cc: joro@8bytes.org; bhelgaas@google.com; agraf@suse.de; Wood Scott-B07421;
> > > Yoder Stuart-B08248; iommu@lists.linux-foundation.org; linux-
> > > pci@vger.kernel.org; linuxppc-dev@lists.ozlabs.org; linux-
> > > kernel@vger.kernel.org; Bhushan Bharat-R65777
> > > Subject: Re: [PATCH 0/9 v2] vfio-pci: add support for Freescale IOMMU (PAMU)
> > > 
> > > Is VFIO_IOMMU_PAMU_GET_MSI_BANK_COUNT per aperture (ie. each vfio user has
> > > $COUNT regions at their disposal exclusively)?
> > 
> > Number of msi-bank count is system wide and not per aperture, But will be setting windows for banks in the device aperture.
> > So say if we are direct assigning 2 pci device (both have different iommu group, so 2 aperture in iommu) to VM.
> > Now qemu can make only one call to know how many msi-banks are there but it must set sub-windows for all banks for both pci device in its respective aperture.
> 
> I'm still confused.  What I want to make sure of is that the banks are
> independent per aperture.  For instance, if we have two separate
> userspace processes operating independently and they both chose to use
> msi bank zero for their device, that's bank zero within each aperture
> and doesn't interfere.  Or another way to ask is can a malicious user
> interfere with other users by using the wrong bank.  Thanks,

They can interfere.  With this hardware, the only way to prevent that is
to make sure that a bank is not shared by multiple protection contexts.
For some of our users, though, I believe preventing this is less
important than the performance benefit.

-Scott