qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [RFC PATCH V4 2/4] vfio: Add vm status change callback


From: Alex Williamson
Subject: Re: [Qemu-devel] [RFC PATCH V4 2/4] vfio: Add vm status change callback to stop/restart the mdev device
Date: Tue, 17 Apr 2018 14:09:03 -0600

On Wed, 18 Apr 2018 00:44:35 +0530
Kirti Wankhede <address@hidden> wrote:

> 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?

Yes, ioctls are something that should be feared and reviewed with great
scrutiny and we should feel bad if we do a poor job defining them and
burn ioctl numbers whereas we have 32bits worth of region sub-types
and 31 bits of region types to churn through within our own address
space and we can easily deprecate losing designs without much harm.
Thus, I want to see that an ioctl is really the best way to perform the
task rather than just being the default answer to everything.  Is it
really a problem if data starts at some offset into the region?  That
sounds like part of the region API that I want to see defined.  The
region can start with a header containing explicit save state version
and device information, writable registers for relaying state
information, an offset to the start of the vendor data field, etc.  If
we can make a GPU work via a definition of registers and doorbells and
framebuffers in an MMIO region then surely we can figure out a virtual
register and buffer definition to do save and restore of the device.
Otherwise, why is an ioctl the best tool for this task?

> >>>> 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?

Why?  CPU state is describing something disassociated with the device.
QEMU will interpret the CPU state into something it wants the device to
do.  The VFIO interface should be defined in terms of the state you
want to impose on the device.  What the CPU is doing is not the device's
problem.  Make sense?  Thanks,

Alex



reply via email to

[Prev in Thread] Current Thread [Next in Thread]