Re: [Qemu-devel] Assigning an eth port to a guest VM

[Alex Williamson]

On Mon, 2015-06-15 at 17:45 +0000, Yehuda Yitschak wrote:
> ________________________________________
> From: Alex Williamson <address@hidden>
> Sent: Monday, June 15, 2015 8:15 PM
> To: Yehuda Yitschak
> Cc: Eric Auger; address@hidden; Yuval Caduri; Shadi Ammouri
> Subject: Re: Assigning an eth port to a guest VM
> 
> On Mon, 2015-06-15 at 16:52 +0000, Yehuda Yitschak wrote:
> >> ________________________________________
> >> From: Eric Auger <address@hidden>
> >> Sent: Monday, June 15, 2015 4:42 PM
> >> To: Yehuda Yitschak; address@hidden
> >> Cc: Yuval Caduri; Shadi Ammouri
> >> Subject: Re: Assigning an eth port to a guest VM
> >>
> >> Hi Yehuda,
> >> On 06/15/2015 01:01 PM, Yehuda Yitschak wrote:
> >> >> Cc: Eric Auger
> >> >>
> >> >>> -----Original Message-----
> >> >>> From: Yehuda Yitschak
> >> >>> Sent: Monday, June 15, 2015 9:35
> >> >>> To: address@hidden
> >> >>> Cc: Yuval Caduri; Shadi Ammouri
> >> >>> Subject: Assigning an eth port to a guest VM
> >> >>>
> >> >>> Hello
> >> >>>
> >> >>> I would to ask your advice on how to assign a semi-virtualized 
> >> >>> Ethernet port
> >> >>> to a guest VM
> >> >>>
> >> >>> The eth port's HW partially supports virtualization since the data 
> >> >>> path MMIO
> >> >>> registers (which controls rx/tx operation) are duplicated per VM.
> >> >>> So for the run-time operation the guest can directly access the MMIO
> >> >>> registers, using VFIO-PLATFORM, and enjoy the performance benefit.
> >> >>>
> >> >>> However for the initial setup and occasional configuration the guest 
> >> >>> need to
> >> >>> access control path registers which are shared for all guests.
> >> >>> AFAIK this is usually done with HW emulation using trap & emulate with
> >> >>> QEMU.
> >> >>> So, to the best of my knowledge I need a mix of VFIO and HW emulation 
> >> >>> to
> >> >>> get the port to work with device assignment , right ?
> >> > Yes to me you're correct.
> >> >>>
> >> >>> Are there any standard methods for achieving this ?
> >> >>> Is there an example for such an existing HW in QEMU ?
> >> > Not yet unfortunately. To my knowledge the only platform devices that
> >> > were assigned with QEMU VFIO platform were standalone duplicated
> >> > devices, PL330, Calxeda Xgmac, SATA. So you are a trailblazer on that
> >> > track.
> >>
> >> Thanks. It's good to know the diagnosis :-)
> >>
> >> BTW - i thought SR-IOV uses a somewhat similar concept. AFAIK each virtual 
> >> function (VF) gets
> >> a set of registers enabling it to perform data path but most of the 
> >> configuration and management
> >> operations are controlled by the host using the Physical Function PF 
> >> driver.
> >> Are you familiar with that ?
> >> i know SR-IOV is not related to VFIO-PLATFORM but if the mixed of direct 
> >> access and emulation
> >> exists there as well then maybe i can borrow some concepts
> 
> > The difference for SR-IOV is that emulation of shared resources is done
> > almost entirely in the hardware.  the PF configures the VFs and may
> >interact with them to some degree at runtime, but VFs are largely
> >separate devices from a software perspective.
> 
> > The first question I would have for your device is whether there is
> > IOMMU isolation between the individual "functions".  
> 
> Yes. IOMMU isolation is possible. 
> 
> > If not, there's really nothing vfio can help with and they probably ought 
> > to be used
> > more as a macvtap interface.  If there is isolation, then I'd assume
> > we'd configure the device for direct access to the duplicated registers
> > and trap to QEMU for the emulation portion.  For things were the
> > emulation portion needs to interact with the "PF", interfaces would need
> > to be created in the kernel.
> 
> Can you give a short example of such an interface ? 
> Do you mean a special device or ioctl to handle the emulation request from 
> QEMU/VFIO ?

It's a trivial example, but with PCI we have a configuration space where
the first 4 bytes expose the vendor and device ID of the device.  With
an SR-IOV VF, these bytes are not populated and provided instead by the
PF via the SR-IOV capability definition on the PF.  The vfio-pci driver
therefore exposes the static PF defined vendor and device IDs though the
VF config space.  It's transparent to the user.

I would hope we wouldn't need any sort of special device or ioctl.  It
sounds like the "PF" registers are separate and distinct from the "VF"
registers, so the "PF" registers could be exposed through a separate
VFIO memory region that does not allow mmap, forcing them to be trapped
into QEMU and emulated in VFIO.

> > The vfio-platform pieces specific to your
> > device might be the logical place for that interaction with the PF to
> > occur, ie. emulation at the vfio-platform interface rather than in QEMU
> > itself.  Thanks,
> 
> That sounds simpler than adding QEMU to the mix. 
> However for that to happen we need to trap into the vfio-platfrom driver, 
> right ? 
> is that possible ? 

Yes.  The vfio-platform driver specific to this device would expose a
memory region for those "VF" registers that does not allow mmap.  The
only access would be via read/write handlers.  You could then
emulate/gate/police access to those registers on the "PF" using kernel
internal interfaces.  It would be a kernel internal API for accessing
the PF registers.  Thanks,

Alex