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

[Neo Jia]

On Wed, Jun 08, 2016 at 11:18:42AM +0800, Dong Jia wrote:
> On Tue, 7 Jun 2016 19:39:21 -0600
> Alex Williamson <address@hidden> wrote:
> 
> > On Wed, 8 Jun 2016 01:18:42 +0000
> > "Tian, Kevin" <address@hidden> wrote:
> > 
> > > > 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.  
> Dear Alex, Kevin and Neo,
> 
> 'ssch' is a privileged I/O instruction, which should be finally issued
> to the dedicated subchannel of the physical device.
> 
> BTW, I did remove step Q2 with all of the user-space translation code,
> according to your comments in another thread.
> 
> > > 
> > > 
> > > >   
> > > > > 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.
> We are different here. The target of an I/O instruction is the
> subchannel. CCW devices don't have these kind of registers. The mediated
> ccw device can not recognize such an submission by its own capbilities. 
> 
> A CCW device does not have such registers in both the physical and the
> mediated devices to sense or recognize the submission request. It's the
> CPU that recognizes the submission by intercepting the guest ssch
> instruction.
> 
> CPU can not tell if it is issued from a passed thru device driver or a
> virtio device driver from the guest. So it has to exit to QEMU, and let
> QEMU take over.

Hi Dong,

What actually has triggered the VM_EXIT to QEMU of that vCPU? Is it an MMIO
access of the "virtual device" inside guest?

Thanks,
Neo

> 
> Once QEMU identifies the target subchannel is serving a passed thru
> device, it uses the ioctl to pass the instruction parameters into the
> kernel all the way along the mediated driver to the physical driver to
> the subchannel to perform the I/O operation.
> 
> > > 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.
> We don't have registers to sense an instruction or operation.
> 
> > 
> > I think we're in violent agreement ;)
> > 
> 
> --------
> Dong Jia
>