Re: [Qemu-devel] Question: SRIOV support over Win Hyper-V VM running in QEMU process on Linux host

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 26, 2018 at 10:42:52AM -0600, Alex Williamson wrote:
> On Thu, 26 Jul 2018 19:14:45 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Thu, Jul 26, 2018 at 06:51:13PM +0300, Marcel Apfelbaum wrote:
> > > Hi
> > > 
> > > On 07/26/2018 05:52 PM, Stefan Hajnoczi wrote:  
> > > > On Thu, Jul 12, 2018 at 07:33:14AM +0000, Elijah Shakkour wrote:  
> > > > > Hey,
> > > > > 
> > > > > Our team is adding a NIC functional  emulation to QEMU.
> > > > > One of the features we are adding to this NIC is SRIOV.
> > > > > 
> > > > > Here is the error message I get when checking SRIOV support of  our emulated NIC on Win2016 server (the hyper-v VM).
> > > > > "
> > > > > SR-IOV cannot be used on this system as the PCI Express hardware does not support Access Control Services (ACS) at any root port.  
> > > 
> > > QEMU's emulated PCI Express Root Ports do not support ACS yet, however I am
> > > not sure ACS is a prerequisite
> > > for SR-IOV.
> 
> ACS is certainly not a prerequisite for SR-IOV.
>  
> > Looks like windows blocks dev assignment in nested VMs without it.
> > Thinking about it, doesn't vfio do the same by default? I think vfio has
> > a flag to override this though.
> 
> IOMMU grouping in Linux takes isolation via ACS and device specific
> mechanisms into account, limiting the granularity with which a
> userspace driver can claim ownership of devices,

In that you must assign all devices behind this port as a group?

> but it doesn't
> actually prevent enabling SR-IOV on the endpoint.  It just makes it
> less useful if your intention is to use SR-IOV for device assignment.
> The only overrides for this are out-of-tree.  Thanks,
> 
> Alex