Re: [Xen-devel] [iGVT-g] [vfio-users] [PATCH v3 00/11] igd passthrough chipset tweaks

["Kay, Allen M" <allen.m.kay@xxxxxxxxx>]

> -----Original Message-----
> From: Tian, Kevin
> Sent: Monday, February 01, 2016 11:08 PM
> To: Kay, Allen M; Alex Williamson; Gerd Hoffmann; qemu-devel@xxxxxxxxxx
> Cc: igvt-g@xxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx; Eduardo Habkost;
> Stefano Stabellini; Cao jin; vfio-users@xxxxxxxxxx
> Subject: RE: [iGVT-g] [vfio-users] [PATCH v3 00/11] igd passthrough chipset
> tweaks
> 
> > From: Kay, Allen M
> > Sent: Saturday, January 30, 2016 5:58 AM
> >
> > First of all, I would like to clarify I'm talking about general IGD
> > passthrough case - not specific to KVMGT.  In IGD passthrough
> > configuration, one of the following will happen when the driver accesses
> OpRegion:
> >
> > 1) If the hypervisor sets up OpRegion GPA/HPA mapping, either by
> > pre-map it (i.e. Xen) or map it during EPT page fault (i.e. KVM),
> > guest can successfully read the content of the OpRegion and check the ID
> string.  In this case, everything works fine.
> >
> > 2) if the hypervisor does not setup OpRegion GPA/HPA mapping at all,
> > then guest driver's attempt to setup GVA/GPA mapping will fail, which
> > causes the driver to fail.  In this case, guest driver won't have the
> > opportunity to look into the content of OpRegion memory and check the ID
> string.
> >
> 
> Guest mapping of GVA->GPA can always succeed regardless of whether
> GPA->HPA is valid. Failure will happen only when the GVA is actually
> accessed by guest.
> 

That is the data from team debugged IGD passthrough on a closed source 
hypervisor that does not map OpRegion with EPT.  The end result is the same 
-driver cannot access inside of OpRegion without failing.

> I don't understand 2). If hypervisor doesn't want to setup mapping, there is
> no chance for guest driver to get opregion content, right?

That was precisely the point I was trying to make.  As a result, guest driver 
needs some indication from the hypervisor that the address at 0xFC contains GPA 
that can be safely accessed by the driver without causing unrecoverable failure 
on hypervisors that does not map OpRegion - by leaving HPA address at 0xFC.

Allen
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