Re: [Qemu-devel] [RFC PATCH V4 2/4] vfio: Add vm status change callback to stop/restart the mdev device

[Kirti Wankhede]

On 4/17/2018 8:13 PM, Alex Williamson wrote:
> On Tue, 17 Apr 2018 13:40:32 +0000
> "Zhang, Yulei" <address@hidden> wrote:
> 
>>> -----Original Message-----
>>> From: Alex Williamson [mailto:address@hidden
>>> Sent: Tuesday, April 17, 2018 4:23 AM
>>> To: Kirti Wankhede <address@hidden>
>>> Cc: Zhang, Yulei <address@hidden>; address@hidden; Tian,
>>> Kevin <address@hidden>; address@hidden;
>>> address@hidden; Wang, Zhi A <address@hidden>;
>>> address@hidden; address@hidden
>>> Subject: Re: [RFC PATCH V4 2/4] vfio: Add vm status change callback to
>>> stop/restart the mdev device
>>>
>>> On Mon, 16 Apr 2018 20:14:27 +0530
>>> Kirti Wankhede <address@hidden> wrote:
>>>   
>>>> On 4/10/2018 11:32 AM, Yulei Zhang wrote:  
>>>>> VM status change handler is added to change the vfio pci device
>>>>> status during the migration, write the demanded device status
>>>>> to the DEVICE STATUS subregion to stop the device on the source side
>>>>> before fetch its status and start the deivce on the target side
>>>>> after restore its status.
>>>>>
>>>>> Signed-off-by: Yulei Zhang <address@hidden>
>>>>> ---
>>>>>  hw/vfio/pci.c                 | 20 ++++++++++++++++++++
>>>>>  include/hw/vfio/vfio-common.h |  1 +
>>>>>  linux-headers/linux/vfio.h    |  6 ++++++
>>>>>  roms/seabios                  |  2 +-
>>>>>  4 files changed, 28 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
>>>>> index f98a9dd..13d8c73 100644
>>>>> --- a/hw/vfio/pci.c
>>>>> +++ b/hw/vfio/pci.c
>>>>> @@ -38,6 +38,7 @@
>>>>>
>>>>>  static void vfio_disable_interrupts(VFIOPCIDevice *vdev);
>>>>>  static void vfio_mmap_set_enabled(VFIOPCIDevice *vdev, bool enabled);
>>>>> +static void vfio_vm_change_state_handler(void *pv, int running,  
>>> RunState state);  
>>>>>
>>>>>  /*
>>>>>   * Disabling BAR mmaping can be slow, but toggling it around INTx can
>>>>> @@ -2896,6 +2897,7 @@ static void vfio_realize(PCIDevice *pdev, Error  
>>> **errp)  
>>>>>      vfio_register_err_notifier(vdev);
>>>>>      vfio_register_req_notifier(vdev);
>>>>>      vfio_setup_resetfn_quirk(vdev);
>>>>> +  
>>> qemu_add_vm_change_state_handler(vfio_vm_change_state_handler,
>>> vdev);  
>>>>>
>>>>>      return;
>>>>>
>>>>> @@ -2982,6 +2984,24 @@ post_reset:
>>>>>      vfio_pci_post_reset(vdev);
>>>>>  }
>>>>>
>>>>> +static void vfio_vm_change_state_handler(void *pv, int running,  
>>> RunState state)  
>>>>> +{
>>>>> +    VFIOPCIDevice *vdev = pv;
>>>>> +    VFIODevice *vbasedev = &vdev->vbasedev;
>>>>> +    uint8_t dev_state;
>>>>> +    uint8_t sz = 1;
>>>>> +
>>>>> +    dev_state = running ? VFIO_DEVICE_START : VFIO_DEVICE_STOP;
>>>>> +
>>>>> +    if (pwrite(vdev->vbasedev.fd, &dev_state,
>>>>> +               sz, vdev->device_state.offset) != sz) {
>>>>> +        error_report("vfio: Failed to %s device", running ? "start" : 
>>>>> "stop");
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>> +    vbasedev->device_state = dev_state;
>>>>> +}
>>>>> +  
>>>>
>>>> Is it expected to trap device_state region by vendor driver?
>>>> Can this information be communicated to vendor driver through an ioctl?  
>>>
>>> Either the mdev vendor driver or vfio bus driver (ie. vfio-pci) would
>>> be providing REGION_INFO for this region, so the vendor driver is
>>> already in full control here using existing ioctls.  I don't see that
>>> we need new ioctls, we just need to fully define the API of the
>>> proposed regions here.
>>>   
>> If the device state region is mmaped, we may not be able to use
>> region device state offset to convey the running state. It may need
>> a new ioctl to set the device state.
> 
> The vendor driver defines the mmap'ability of the region, the vendor
> driver is still in control.  The API of the region and the
> implementation by the vendor driver should account for handling
> mmap'able sections within the region.  Thanks,
> 
> Alex
> 
>  

If this same region should be used for communicating state or other
parameters instead of ioctl, may be first page of this region need to be
reserved. Mmappable region's start address should be page aligned. Is
this API going to utilize 4K of the reserved part of this region?
Instead of carving out part of section from the region, are there any
disadvantages of adding an ioctl?
May be defining a single ioctl and using different flags (GET_*/SET_*)
would work?

>>>> Here only device state is conveyed to vendor driver but knowing
>>>> 'RunState' in vendor driver is very useful and vendor driver can take
>>>> necessary action accordingly like RUN_STATE_PAUSED indicating that VM  
>>> is  
>>>> in paused state, similarly RUN_STATE_SUSPENDED,
>>>> RUN_STATE_FINISH_MIGRATE, RUN_STATE_IO_ERROR. If these states are
>>>> handled properly, all the cases can be supported with same interface
>>>> like VM suspend-resume, VM pause-restore.  
>>>
>>> I agree, but let's remember that we're talking about device state, not
>>> VM state.  vfio is a userspace device interface, not specifically a
>>> virtual machine interface, so states should be in terms of the device.
>>> The API of this region needs to be clearly defined and using only 1
>>> byte at the start of the region is not very forward looking.  Thanks,
>>>
>>> Alex
>>>   

Sorry for using wrong term in my previous reply, 'RunState' is actually
CPU state and not VM state. In terms of vfio device interface knowing
CPU state would be helpful, right?

Thanks,
Kirti

>>>>>  static void vfio_instance_init(Object *obj)
>>>>>  {
>>>>>      PCIDevice *pci_dev = PCI_DEVICE(obj);
>>>>> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-  
>>> common.h  
>>>>> index f3a2ac9..9c14a8f 100644
>>>>> --- a/include/hw/vfio/vfio-common.h
>>>>> +++ b/include/hw/vfio/vfio-common.h
>>>>> @@ -125,6 +125,7 @@ typedef struct VFIODevice {
>>>>>      unsigned int num_irqs;
>>>>>      unsigned int num_regions;
>>>>>      unsigned int flags;
>>>>> +    bool device_state;
>>>>>  } VFIODevice;
>>>>>
>>>>>  struct VFIODeviceOps {
>>>>> diff --git a/linux-headers/linux/vfio.h b/linux-headers/linux/vfio.h
>>>>> index e3380ad..8f02f2f 100644
>>>>> --- a/linux-headers/linux/vfio.h
>>>>> +++ b/linux-headers/linux/vfio.h
>>>>> @@ -304,6 +304,12 @@ struct vfio_region_info_cap_type {
>>>>>  /* Mdev sub-type for device state save and restore */
>>>>>  #define VFIO_REGION_SUBTYPE_DEVICE_STATE (4)
>>>>>
>>>>> +/* Offset in region to save device state */
>>>>> +#define VFIO_DEVICE_STATE_OFFSET 1
>>>>> +
>>>>> +#define VFIO_DEVICE_START        0
>>>>> +#define VFIO_DEVICE_STOP 1
>>>>> +
>>>>>  /**
>>>>>   * VFIO_DEVICE_GET_IRQ_INFO - _IOWR(VFIO_TYPE, VFIO_BASE + 9,
>>>>>   *                                   struct vfio_irq_info)
>>>>> diff --git a/roms/seabios b/roms/seabios
>>>>> index 63451fc..5f4c7b1 160000
>>>>> --- a/roms/seabios
>>>>> +++ b/roms/seabios
>>>>> @@ -1 +1 @@
>>>>> -Subproject commit 63451fca13c75870e1703eb3e20584d91179aebc
>>>>> +Subproject commit 5f4c7b13cdf9c450eb55645f4362ea58fa61b79b
>>>>>  
>>
>