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

From: Tian, Kevin
Subject: Re: [Qemu-devel] [RFC PATCH v4 1/3] Mediated device Core driver
Date: Tue, 7 Jun 2016 03:03:32 +0000

> 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:
> > >
> > > 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:

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
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.

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.


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