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

[Tian, Kevin]

> 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.

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? Or
do you mean some hypervisor wants to emulate the opregion access?
but even in that case there's no failure per se except in a slower path.

Thanks
Kevin