Re: Shouldn't VFIO virtualize the ATS capability?

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 9 Nov 2016 15:25:16 +0000
Ilya Lesokhin <ilyal@mellanox.com> wrote:

> > -----Original Message-----
> > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > Sent: Wednesday, November 09, 2016 5:08 PM
> > To: Ilya Lesokhin <ilyal@mellanox.com>
> > Cc: linux-pci@vger.kernel.org; kvm@vger.kernel.org; bhelgaas@google.com;
> > Adi Menachem <adim@mellanox.com>
> > Subject: Re: Shouldn't VFIO virtualize the ATS capability?
> > 
> > On Wed, 9 Nov 2016 14:49:02 +0000
> > Ilya Lesokhin <ilyal@mellanox.com> wrote:
> >   
> > > I would virtualize the "ATS Control Register".  
> > 
> > And do what?  
> I think it should be read only.

That would violate the spec, in which case it shouldn't be virtualized,
the capability should be hidden.
 
> > > Regarding poor behavior, I couldn't really find what happens when ATS is  
> > misconfigured, but I would assume it can cause problems.  
> > > The scenarios I'm concerned about are:
> > > 	1. The guest enables translation caching, while the hypervisor thinks  
> > there are disabled -> Hypervisor won't issue invalidations.
> > 
> > Aren't invalidations issued by the iommu, why does the hypervisor need to
> > participate?  How would a software entity induce an invalidation?  
> That's what one might expect from a sane design, but
> http://lxr.free-electrons.com/source/drivers/iommu/intel-iommu.c?v=4.8#L1549
> seems to imply otherwise :(
> >   
> > > 	2. Smallest Translation Unit misconfiguration. Not sure if it will cause  
> > invalid access or only poor caching behavior.  
> > >
> > > Thanks,
> > > Ilya
> > >  
> > > > -----Original Message-----
> > > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > > Sent: Sunday, November 06, 2016 7:09 PM
> > > > To: Ilya Lesokhin <ilyal@mellanox.com>
> > > > Cc: linux-pci@vger.kernel.org; kvm@vger.kernel.org;
> > > > bhelgaas@google.com
> > > > Subject: Re: Shouldn't VFIO virtualize the ATS capability?
> > > >
> > > > On Sun, 6 Nov 2016 11:13:09 +0000
> > > > Ilya Lesokhin <ilyal@mellanox.com> wrote:
> > > >  
> > > > > Hi
> > > > > I've noticed that VFIO doesn't virtualize the ATS capability.
> > > > > It seems to me that translation caching and Smallest Translation
> > > > > Unit is  
> > > > something you would want to control on the host. Am I wrong?
> > > >
> > > > What about those fields would we virtualize?  Why does the host need
> > > > to be an intermediary?  Can the user induce poor behavior with
> > > > direct access to them?  Thanks,
> > > >
> > > > Alex  
>