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

[Alex Williamson]

On Thu, 26 Jul 2018 19:14:45 +0300
"Michael S. Tsirkin" <address@hidden> 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, 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