Re: [Qemu-devel] VFIO based vGPU(was Re: [Announcement] 2015-Q3 release of XenGT - a Mediated ...)

[Tian, Kevin]

> From: Alex Williamson [mailto:address@hidden
> Sent: Wednesday, January 27, 2016 12:37 AM
> 
> On Tue, 2016-01-26 at 22:05 +0800, Yang Zhang wrote:
> > On 2016/1/26 15:41, Jike Song wrote:
> > > On 01/26/2016 05:30 AM, Alex Williamson wrote:
> > > > [cc +Neo @Nvidia]
> > > >
> > > > Hi Jike,
> > > >
> > > > On Mon, 2016-01-25 at 19:34 +0800, Jike Song wrote:
> > > > > On 01/20/2016 05:05 PM, Tian, Kevin wrote:
> > > > > > I would expect we can spell out next level tasks toward above
> > > > > > direction, upon which Alex can easily judge whether there are
> > > > > > some common VFIO framework changes that he can help :-)
> > > > >
> > > > > Hi Alex,
> > > > >
> > > > > Here is a draft task list after a short discussion w/ Kevin,
> > > > > would you please have a look?
> > > > >
> > > > >       Bus Driver
> > > > >
> > > > >               { in i915/vgt/xxx.c }
> > > > >
> > > > >               - define a subset of vfio_pci interfaces
> > > > >               - selective pass-through (say aperture)
> > > > >               - trap MMIO: interface w/ QEMU
> > > >
> > > > What's included in the subset?  Certainly the bus reset ioctls really
> > > > don't apply, but you'll need to support the full device interface,
> > > > right?  That includes the region info ioctl and access through the vfio
> > > > device file descriptor as well as the interrupt info and setup ioctls.
> > > >
> > >
> > > [All interfaces I thought are via ioctl:)  For other stuff like file
> > > descriptor we'll definitely keep it.]
> > >
> > > The list of ioctl commands provided by vfio_pci:
> > >
> > >   - VFIO_DEVICE_GET_PCI_HOT_RESET_INFO
> > >   - VFIO_DEVICE_PCI_HOT_RESET
> > >
> > > As you said, above 2 don't apply. But for this:
> > >
> > >   - VFIO_DEVICE_RESET
> > >
> > > In my opinion it should be kept, no matter what will be provided in
> > > the bus driver.
> > >
> > >   - VFIO_PCI_ROM_REGION_INDEX
> > >   - VFIO_PCI_VGA_REGION_INDEX
> > >
> > > I suppose above 2 don't apply neither? For a vgpu we don't provide a
> > > ROM BAR or VGA region.
> > >
> > >   - VFIO_DEVICE_GET_INFO
> > >   - VFIO_DEVICE_GET_REGION_INFO
> > >   - VFIO_DEVICE_GET_IRQ_INFO
> > >   - VFIO_DEVICE_SET_IRQS
> > >
> > > Above 4 are needed of course.
> > >
> > > We will need to extend:
> > >
> > >   - VFIO_DEVICE_GET_REGION_INFO
> > >
> > >
> > > a) adding a flag: DONT_MAP. For example, the MMIO of vgpu
> > > should be trapped instead of being mmap-ed.
> >
> > I may not in the context, but i am curious how to handle the DONT_MAP in
> > vfio driver? Since there are no real MMIO maps into the region and i
> > suppose the access to the region should be handled by vgpu in i915
> > driver, but currently most of the mmio accesses are handled by Qemu.
> 
> VFIO supports the following region attributes:
> 
> #define VFIO_REGION_INFO_FLAG_READ      (1 << 0) /* Region supports read */
> #define VFIO_REGION_INFO_FLAG_WRITE     (1 << 1) /* Region supports write */
> #define VFIO_REGION_INFO_FLAG_MMAP      (1 << 2) /* Region supports mmap */
> 
> If MMAP is not set, then the QEMU driver will do pread and/or pwrite to
> the specified offsets of the device file descriptor, depending on what
> accesses are supported.  This is all reported through the REGION_INFO
> ioctl for a given index.  If mmap is supported, the VM will have direct
> access to the area, without faulting to KVM other than to populate the
> mapping.  Without mmap support, a VM MMIO access traps into KVM, which
> returns out to QEMU to service the request, which then finds the
> MemoryRegion serviced through vfio, which will then perform a
> pread/pwrite through to the kernel vfio bus driver to handle the
> access.  Thanks,
> 

Today KVMGT (not using VFIO yet) registers I/O emulation callbacks to 
KVM, so VM MMIO access will be forwarded to KVMGT directly for 
emulation in kernel. If we reuse above R/W flags, the whole emulation 
path would be unnecessarily long with obvious performance impact. We
either need a new flag here to indicate in-kernel emulation (bias from
passthrough support), or just hide the region alternatively (let KVMGT
to handle I/O emulation itself like today).

Thanks
Kevin