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

[Bharat Bhushan]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Friday, November 22, 2013 2:31 AM
> To: Wood Scott-B07421
> Cc: Bhushan Bharat-R65777; linux-pci@xxxxxxxxxxxxxxx; agraf@xxxxxxx; Yoder
> Stuart-B08248; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; bhelgaas@xxxxxxxxxx; linuxppc-
> dev@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 0/9 v2] vfio-pci: add support for Freescale IOMMU (PAMU)
> 
> On Thu, 2013-11-21 at 14:47 -0600, Scott Wood wrote:
> > 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@xxxxxxxxxx]
> > > > > Sent: Thursday, November 21, 2013 12:17 AM
> > > > > To: Bhushan Bharat-R65777
> > > > > Cc: joro@xxxxxxxxxx; bhelgaas@xxxxxxxxxx; agraf@xxxxxxx; Wood
> > > > > Scott-B07421; Yoder Stuart-B08248;
> > > > > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux- pci@xxxxxxxxxxxxxxx;
> > > > > linuxppc-dev@xxxxxxxxxxxxxxxx; linux- kernel@xxxxxxxxxxxxxxx;
> > > > > 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.

Want to be sure of how 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.

So should we let this patch series in without protection?

> 
> I think we need some sort of ownership model around the msi banks then.
> Otherwise there's nothing preventing another userspace from attempting an MSI
> based attack on other users, or perhaps even on the host.  VFIO can't allow
> that.  Thanks,

We have very few (3 MSI bank on most of chips), so we can not assign one to each userspace. What we can do is host and userspace does not share a MSI bank while userspace will share a MSI bank.


Thanks
-Bharat

> 
> Alex
> 

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