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Re: [Qemu-devel] [PATCH v8 11/12] vfio: register aer resume notification

From: Zhou Jie
Subject: Re: [Qemu-devel] [PATCH v8 11/12] vfio: register aer resume notification handler for aer resume
Date: Sat, 25 Jun 2016 09:24:19 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0

Hi Alex,

We should never depend on the guest driver to behave in a certain way,
but we need to prioritize what that actually means.  vfio in the kernel
has a responsibility first and foremost to the host kernel.  User owned
devices cannot be allowed to exploit or interfere with the host
regardless of user behavior.  The next priority is correct operation
for the user.  When the host kernel is handling the AER event between
the error and resume notifies, it doesn't have device specific drivers,
it's manipulating the device as a generic PCI device.  That makes me
think that vfio should not allow the user to interact (interfere) with
the device during that process and that such interference can be
limited to standard PCI level interactions.  That means config space,
and things that operate on config space (like interrupt ioctls and
resets).  On the QEMU side, we've sent a notification that an error
occurred, how the user and the guest respond to that is beyond the
concern of vfio in the kernel.  If the user/guest driver continues to
interact with resources on the device, that's fine, but I think vfio in
the kernel does need to prevent the user from interfering with the PCI
state of the device for that brief window when we know the host kernel
is operating on the device.  Otherwise the results are unpredictable
and therefore unsupportable.  Does that make sense?  Thanks,
I understand.

I want to alter the VFIO driver like following.
During err occurs and resume:
1. Make config space read only.
   Ignore config space writing to prevent the user from
   interfering with the PCI state of the device.
   User can get the error infomation by reading the config space.
2. Disable INTx and MSI
   Write "Command Register" to disable INTx and MSI.
3. Do nothing for bar regions.
   Guest driver may access bar regions.
   It doesn't matter as device is going to be reset.

The following code will be modified.
1. vfio_pci_ioctl
   add flag for aer support
2. vfio_pci_ioctl
   During err occurs and resume:
   if (cmd == VFIO_DEVICE_SET_IRQS) return EAGAIN
   if (cmd == VFIO_DEVICE_RESET) return EAGAIN
   if (cmd == VFIO_DEVICE_PCI_HOT_RESET) return EAGAIN
3. vfio_pci_write
   During err occurs and resume:
4. vfio_pci_aer_err_detected
   Set aer state in "struct vfio_pci_device"
   Write "Command Register" to disable INTx and MSI.
5. vfio_pci_aer_resume
   Clear aer state in "struct vfio_pci_device"
   I don't need to enable INTx and MSI.
   The device will be initalized by guest driver.


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