Re: [Qemu-devel] [RFC PATCH v4 1/3] Mediated device Core driver

[Tian, Kevin]

> From: Alex Williamson [mailto:address@hidden
> Sent: Wednesday, June 08, 2016 6:42 AM
> 
> On Tue, 7 Jun 2016 03:03:32 +0000
> "Tian, Kevin" <address@hidden> wrote:
> 
> > > From: Alex Williamson [mailto:address@hidden
> > > Sent: Tuesday, June 07, 2016 3:31 AM
> > >
> > > On Mon, 6 Jun 2016 10:44:25 -0700
> > > Neo Jia <address@hidden> wrote:
> > >
> > > > On Mon, Jun 06, 2016 at 04:29:11PM +0800, Dong Jia wrote:
> > > > > On Sun, 5 Jun 2016 23:27:42 -0700
> > > > > Neo Jia <address@hidden> wrote:
> > > > >
> > > > > 2. VFIO_DEVICE_CCW_CMD_REQUEST
> > > > > This intends to handle an intercepted channel I/O instruction. It
> > > > > basically need to do the following thing:
> > > >
> > > > May I ask how and when QEMU knows that he needs to issue such VFIO 
> > > > ioctl at
> > > > first place?
> > >
> > > Yep, this is my question as well.  It sounds a bit like there's an
> > > emulated device in QEMU that's trying to tell the mediated device when
> > > to start an operation when we probably should be passing through
> > > whatever i/o operations indicate that status directly to the mediated
> > > device. Thanks,
> > >
> > > Alex
> >
> > Below is copied from Dong's earlier post which said clear that
> > a guest cmd submission will trigger the whole flow:
> >
> > ----
> > Explanation:
> > Q1-Q4: Qemu side process.
> > K1-K6: Kernel side process.
> >
> > Q1. Intercept a ssch instruction.
> > Q2. Translate the guest ccw program to a user space ccw program
> >     (u_ccwchain).
> > Q3. Call VFIO_DEVICE_CCW_CMD_REQUEST (u_ccwchain, orb, irb).
> >     K1. Copy from u_ccwchain to kernel (k_ccwchain).
> >     K2. Translate the user space ccw program to a kernel space ccw
> >         program, which becomes runnable for a real device.
> >     K3. With the necessary information contained in the orb passed in
> >         by Qemu, issue the k_ccwchain to the device, and wait event q
> >         for the I/O result.
> >     K4. Interrupt handler gets the I/O result, and wakes up the wait q.
> >     K5. CMD_REQUEST ioctl gets the I/O result, and uses the result to
> >         update the user space irb.
> >     K6. Copy irb and scsw back to user space.
> > Q4. Update the irb for the guest.
> > ----
> 
> Right, but this was the pre-mediated device approach, now we no longer
> need step Q2 so we really only need Q1 and therefore Q3 to exist in
> QEMU if those are operations that are not visible to the mediated
> device; which they very well might be, since it's described as an
> instruction rather than an i/o operation.  It's not terrible if that's
> the case, vfio-pci has its own ioctl for doing a hot reset.



> 
> > My understanding is that such thing belongs to how device is mediated
> > (so device driver specific), instead of something to be abstracted in
> > VFIO which manages resource but doesn't care how resource is used.
> >
> > Actually we have same requirement in vGPU case, that a guest driver
> > needs submit GPU commands through some MMIO register. vGPU device
> > model will intercept the submission request (in its own way), do its
> > necessary scan/audit to ensure correctness/security, and then submit
> > to physical GPU through vendor specific interface.
> >
> > No difference with channel I/O here.
> 
> Well, if the GPU command is submitted through an MMIO register, is that
> MMIO register part of the mediated device?  If so, could the mediated
> device recognize the command and do the scan/audit itself?  QEMU must
> not be the point at which mediation occurs for security purposes, QEMU
> is userspace and userspace is not to be trusted.  I'm still open to
> ioctls where it makes sense, as above, we have PCI specific ioctls and
> already, but we need to evaluate each one, why it needs to exist, and
> whether we can skip it if the mediated device can trigger the action on
> its own.  After all, that's why we're using the vfio api, so we can
> re-use much of the existing infrastructure, especially for a vGPU that
> exposes itself as a PCI device.  Thanks,
> 

My point is that a guest submission on vGPU is just a normal trapped 
register write, which is forwarded from Qemu to VFIO through pwrite 
interface and then hit mediated vGPU device. The mediated device
will recognize this register write as a submission request and then do
necessary scan (looks we are saying same thing) and then submit to
physical device driver. If loading ccw cmds on channel i/o are also 
through some I/O registers, it can be implemented same way w/o
introducing new ioctl. The r/w handler of mediated device can figure
out whether it's a ccw submission or not. But my understanding might 
be wrong here.

Thanks
Kevin