Re: [Xen-devel] PCIe IOMMU ACS support

[Roman Shaposhnik <roman@xxxxxxxxxx>]

On Fri, Mar 27, 2020 at 2:12 AM Paul Durrant <xadimgnik@xxxxxxxxx> wrote:
>
> > -----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, this is *exactly* what I was asking about -- thanks for the link.

I guess the only question I have left is whether there was any follow up
regarding what you sketched out in the "IOMMU drivers" section?

Thanks,
Roman.