Re: [PATCH V7 19/29] vfio-pci: cpr part 1 (fd and dma)

[Michael S. Tsirkin]

On Wed, Jan 05, 2022 at 04:40:43PM -0500, Steven Sistare wrote:
> On 1/5/2022 4:14 PM, Michael S. Tsirkin wrote:
> > On Wed, Jan 05, 2022 at 12:24:21PM -0500, Steven Sistare wrote:
> >> On 12/22/2021 6:15 PM, Michael S. Tsirkin wrote:
> >>> On Wed, Dec 22, 2021 at 11:05:24AM -0800, Steve Sistare wrote:
> >>>> Enable vfio-pci devices to be saved and restored across an exec restart
> >>>> of qemu.
> >>>>
> >>>> At vfio creation time, save the value of vfio container, group, and 
> >>>> device
> >>>> descriptors in cpr state.
> >>>>
> >>>> In cpr-save and cpr-exec, suspend the use of virtual addresses in DMA
> >>>> mappings with VFIO_DMA_UNMAP_FLAG_VADDR, because guest ram will be 
> >>>> remapped
> >>>> at a different VA after exec.  DMA to already-mapped pages continues.  
> >>>> Save
> >>>> the msi message area as part of vfio-pci vmstate, save the interrupt and
> >>>> notifier eventfd's in cpr state, and clear the close-on-exec flag for the
> >>>> vfio descriptors.  The flag is not cleared earlier because the 
> >>>> descriptors
> >>>> should not persist across miscellaneous fork and exec calls that may be
> >>>> performed during normal operation.
> >>>>
> >>>> On qemu restart, vfio_realize() finds the saved descriptors, uses
> >>>> the descriptors, and notes that the device is being reused.  Device and
> >>>> iommu state is already configured, so operations in vfio_realize that
> >>>> would modify the configuration are skipped for a reused device, including
> >>>> vfio ioctl's and writes to PCI configuration space.  The result is that
> >>>> vfio_realize constructs qemu data structures that reflect the current
> >>>> state of the device.  However, the reconstruction is not complete until
> >>>> cpr-load is called. cpr-load loads the msi data and finds eventfds in cpr
> >>>> state.  It rebuilds vector data structures and attaches the interrupts to
> >>>> the new KVM instance.  cpr-load then invokes the main vfio listener 
> >>>> callback,
> >>>> which walks the flattened ranges of the vfio_address_spaces and calls
> >>>> VFIO_DMA_MAP_FLAG_VADDR to inform the kernel of the new VA's.  Lastly, it
> >>>> starts the VM and suppresses vfio pci device reset.
> >>>>
> >>>> This functionality is delivered by 3 patches for clarity.  Part 1 handles
> >>>> device file descriptors and DMA.  Part 2 adds eventfd and MSI/MSI-X 
> >>>> vector
> >>>> support.  Part 3 adds INTX support.
> >>>>
> >>>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> >>>> ---
> >>>>  MAINTAINERS                   |   1 +
> >>>>  hw/pci/pci.c                  |  10 ++++
> >>>>  hw/vfio/common.c              | 115 
> >>>> ++++++++++++++++++++++++++++++++++++++----
> >>>>  hw/vfio/cpr.c                 |  94 ++++++++++++++++++++++++++++++++++
> >>>>  hw/vfio/meson.build           |   1 +
> >>>>  hw/vfio/pci.c                 |  77 ++++++++++++++++++++++++++++
> >>>>  hw/vfio/trace-events          |   1 +
> >>>>  include/hw/pci/pci.h          |   1 +
> >>>>  include/hw/vfio/vfio-common.h |   8 +++
> >>>>  include/migration/cpr.h       |   3 ++
> >>>>  migration/cpr.c               |  10 +++-
> >>>>  migration/target.c            |  14 +++++
> >>>>  12 files changed, 324 insertions(+), 11 deletions(-)
> >>>>  create mode 100644 hw/vfio/cpr.c
> >>>>
> >>>> diff --git a/MAINTAINERS b/MAINTAINERS
> >>>> index cfe7480..feed239 100644
> >>>> --- a/MAINTAINERS
> >>>> +++ b/MAINTAINERS
> >>>> @@ -2992,6 +2992,7 @@ CPR
> >>>>  M: Steve Sistare <steven.sistare@oracle.com>
> >>>>  M: Mark Kanda <mark.kanda@oracle.com>
> >>>>  S: Maintained
> >>>> +F: hw/vfio/cpr.c
> >>>>  F: include/migration/cpr.h
> >>>>  F: migration/cpr.c
> >>>>  F: qapi/cpr.json
> >>>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> >>>> index 0fd21e1..e35df4f 100644
> >>>> --- a/hw/pci/pci.c
> >>>> +++ b/hw/pci/pci.c
> >>>> @@ -307,6 +307,16 @@ static void pci_do_device_reset(PCIDevice *dev)
> >>>>  {
> >>>>      int r;
> >>>>  
> >>>> +    /*
> >>>> +     * A reused vfio-pci device is already configured, so do not reset 
> >>>> it
> >>>> +     * during qemu_system_reset prior to cpr-load, else interrupts may 
> >>>> be
> >>>> +     * lost.  By contrast, pure-virtual pci devices may be reset here 
> >>>> and
> >>>> +     * updated with new state in cpr-load with no ill effects.
> >>>> +     */
> >>>> +    if (dev->reused) {
> >>>> +        return;
> >>>> +    }
> >>>> +
> >>>>      pci_device_deassert_intx(dev);
> >>>>      assert(dev->irq_state == 0);
> >>>>  
> >>>
> >>>
> >>> Hmm that's a weird thing to do. I suspect this works because
> >>> "reused" means something like "in the process of being restored"?
> >>> Because clearly, we do not want to skip this part e.g. when
> >>> guest resets the device.
> >>
> >> Exactly.  vfio_realize sets the flag if it detects the device is reused 
> >> during
> >> a restart, and vfio_pci_post_load clears the reused flag.
> >>
> >>> So a better name could be called for, but really I don't
> >>> love how vfio gets to poke at internal PCI state.
> >>> I'd rather we found a way just not to call this function.
> >>> If we can't, maybe an explicit API, and make it
> >>> actually say what it's doing?
> >>
> >> How about:
> >>
> >> pci_set_restore(PCIDevice *dev) { dev->restore = true; }
> >> pci_clr_restore(PCIDevice *dev) { dev->restore = false; }
> >>
> >> vfio_realize()
> >>   pci_set_restore(pdev)
> >>
> >> vfio_pci_post_load()
> >>   pci_clr_restore(pdev)
> >>
> >> pci_do_device_reset()
> >>     if (dev->restore)
> >>         return;
> >>
> >> - Steve
> > 
> > 
> > Not too bad. I'd like a better definition of what dev->restore is
> > exactly and to add them in comments near where it
> > is defined and used.
> 
> Will do.
> 
> > E.g. does this mean "device is being restored because of qemu restart"?
> > 
> > Do we need a per device flag for this thing or would a global
> > "qemu restart in progress" flag be enough?
> 
> A global flag (or function, which already exists) would suppress reset for all
> PCI devices, not just vfio-pci.  I am concerned that for some devices, 
> vmstate 
> load may implicitly depend on the device having been reset for correctness, 
> by 
> virtue of some fields being initialized in the reset function.
> 
> - Steve

So just so I understand, how do these other devices work with restart?
Do they use the save/loadvm machinery? And the reason vfio doesn't
is because it generally does not support savevm/loadvm?

-- 
MST