Re: [Xen-devel] PCIe IOMMU ACS support

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Roman Shaposhnik <roman@xxxxxxxxxx>
> Sent: 26 March 2020 22:03
> To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Jan Beulich <jbeulich@xxxxxxxx>; Paul 
> Durrant <paul@xxxxxxx>;
> Kevin Tian <kevin.tian@xxxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Subject: Re: [Xen-devel] PCIe IOMMU ACS support
> 
> On Wed, Mar 25, 2020 at 4:05 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
> >
> > Adding the PCI and IOMMU maintainers.
> >
> > On Mon, Mar 23, 2020 at 01:55:01PM -0700, Roman Shaposhnik wrote:
> > > Hi!
> > >
> > > I was going through how Xen support PCIe IOMMU ACS and
> > > all I could find is this:
> > >     
> > > https://github.com/xen-project/xen/blob/master/xen/drivers/passthrough/pci.c#L608
> > > which looks to me as an attempt of enabling ACS opportunistically,
> > > but still proceeding forward even if it fails.
> >
> > That's correct AFAICT. Xen will try to enable some features, but will
> > proceed normally if ACS is not available, or if some of the features
> > are not implemented.
> >
> > Are you looking to ensure that all devices on the system have a
> > certain feature enabled?
> 
> My primary objective was to get some visibility into how Xen would
> prevent two PCIe devices behind a common bridge from doing p2p
> transactions (thus violating VM boundaries if those devices are
> assigned to different domains).
> 
> It looks like Xen simply trusts the hardware.
> 
> > Can you provide some more details about what you expect of ACS
> > handling?
> 
> I was actually surprised not to see IOMMU groups in the style of what
> VFIO https://www.kernel.org/doc/Documentation/vfio.txt
> 

I did write a doc some time ago to present the issues facing Xen w.r.t. IOMMU 
and device pass-through. Hopefully you can see it at 
https://docs.google.com/document/d/12-z6JD41J_oNrCg_c0yAxGWg5ADBQ8_bSiP_NH6Hqwo/edit?usp=sharing

  Paul