Re: [PATCH V1 30/32] vfio-pci: save and restore

[Jason Zeng]

On Wed, Aug 19, 2020 at 05:15:11PM -0400, Steven Sistare wrote:
> On 8/9/2020 11:50 PM, Jason Zeng wrote:
> > On Fri, Aug 07, 2020 at 04:38:12PM -0400, Steven Sistare wrote:
> >> On 8/6/2020 6:22 AM, Jason Zeng wrote:
> >>> Hi Steve,
> >>>
> >>> On Thu, Jul 30, 2020 at 08:14:34AM -0700, Steve Sistare wrote:
> >>>> @@ -3182,6 +3207,51 @@ static Property vfio_pci_dev_properties[] = {
> >>>>      DEFINE_PROP_END_OF_LIST(),
> >>>>  };
> >>>>  
> >>>> +static int vfio_pci_post_load(void *opaque, int version_id)
> >>>> +{
> >>>> +    int vector;
> >>>> +    MSIMessage msg;
> >>>> +    Error *err = 0;
> >>>> +    VFIOPCIDevice *vdev = opaque;
> >>>> +    PCIDevice *pdev = &vdev->pdev;
> >>>> +
> >>>> +    if (msix_enabled(pdev)) {
> >>>> +        vfio_msix_enable(vdev);
> >>>> +        pdev->msix_function_masked = false;
> >>>> +
> >>>> +        for (vector = 0; vector < vdev->pdev.msix_entries_nr; vector++) 
> >>>> {
> >>>> +            if (!msix_is_masked(pdev, vector)) {
> >>>> +                msg = msix_get_message(pdev, vector);
> >>>> +                vfio_msix_vector_use(pdev, vector, msg);
> >>>> +            }
> >>>> +        }
> >>>
> >>> It looks to me MSIX re-init here may lose device IRQs and impact
> >>> device hardware state?
> >>>
> >>> The re-init will cause the kernel vfio driver to connect the device
> >>> MSIX vectors to new eventfds and KVM instance. But before that, device
> >>> IRQs will be routed to previous eventfd. Looks these IRQs will be lost.
> >>
> >> Thanks Jason, that sounds like a problem.  I could try reading and saving 
> >> an 
> >> event from eventfd before shutdown, and injecting it into the eventfd after
> >> restart, but that would be racy unless I disable interrupts.  Or, 
> >> unconditionally
> >> inject a spurious interrupt after restart to kick it, in case an interrupt 
> >> was lost.
> >>
> >> Do you have any other ideas?
> > 
> > Maybe we can consider to also hand over the eventfd file descriptor, or
> 
> I believe preserving this descriptor in isolation is not sufficient.  We would
> also need to preserve the KVM instance which it is linked to.
> 
> > or even the KVM fds to the new Qemu?
> > 
> > If the KVM fds can be preserved, we will just need to restore Qemu KVM
> > side states. But not sure how complicated the implementation would be.
> 
> That should work, but I fear it would require many code changes in QEMU
> to re-use descriptors at object creation time and suppress the initial 
> configuration ioctl's, so it's not my first choice for a solution.
> 
> > If we only preserve the eventfd fd, we can attach the old eventfd to
> > vfio devices. But looks it may turn out we always inject an interrupt
> > unconditionally, because kernel KVM irqfd eventfd handling is a bit
> > different than normal user land eventfd read/write. It doesn't decrease
> > the counter in the eventfd context. So if we read the eventfd from new
> > Qemu, it looks will always have a non-zero counter, which requires an
> > interrupt injection.
> 
> Good to know, thanks.
> 
> I will try creating a new eventfd and injecting an interrupt unconditionally.
> I need a test case to demonstrate losing an interrupt, and fixing it with
> injection.  Any advice?  My stress tests with a virtual function nic and a
> directly assigned nvme block device have never failed across live update.
> 

I am not familiar with nvme devices. For nic device, to my understanding,
stress nic testing will not have many IRQs, because nic driver usually
enables NAPI, which only take the first interrupt, then disable interrupt
and start polling. It will only re-enable interrupt after some packet
quota reached or the traffic quiesces for a while. But anyway, if the
test goes enough long time, the number of IRQs should also be big, not
sure why it doesn't trigger any issue. Maybe we can have some study on
the IRQ pattern for the testing and see how we can design a test case?
or see if our assumption is wrong?


> >>> And the re-init will make the device go through the procedure of
> >>> disabling MSIX, enabling INTX, and re-enabling MSIX and vectors.
> >>> So if the device is active, its hardware state will be impacted?
> >>
> >> Again thanks.  vfio_msix_enable() does indeed call 
> >> vfio_disable_interrupts().
> >> For a quick experiment, I deleted that call in for the post_load code 
> >> path, and 
> >> it seems to work fine, but I need to study it more.
> > 
> > vfio_msix_vector_use() will also trigger this procedure in the kernel.
> 
> Because that code path calls VFIO_DEVICE_SET_IRQS? Or something else?
> Can you point to what it triggers in the kernel?


In vfio_msix_vector_use(), I see vfio_disable_irqindex() will be invoked
if "vdev->nr_vectors < nr + 1" is true. Since the 'vdev' is re-inited,
so this condition should be true, and vfio_disable_irqindex() will
trigger VFIO_DEVICE_SET_IRQS with VFIO_IRQ_SET_DATA_NONE, which will
cause kernel to disable MSIX.

> 
> > Looks we shouldn't trigger any kernel vfio actions here? Because we
> > preserve vfio fds, so its kernel state shouldn't be touched. Here we
> > may only need to restore Qemu states. Re-connect to KVM instance should
> > be done automatically when we setup the KVM irqfds with the same eventfd.
> > 
> > BTW, if I remember correctly, it is not enough to only save MSIX state
> > in the snapshot. We should also save the Qemu side pci config space
> > cache to the snapshot, because Qemu's copy is not exactly the same as
> > the kernel's copy. I encountered this before, but I don't remember which
> > field it was.
> 
> FYI all, Jason told me offline that qemu may emulate some pci capabilities and
> hence keeps state in the shadow config that is never written to the kernel.
> I need to study that.
> 

Sorry, I read the code again, see Qemu does write all config-space-write
to kernel in vfio_pci_write_config(). Now I am also confused about what
I was seeing previously :(. But it seems we still need to look at kernel
code to see if mismatch is possibile for config space cache between Qemu
and kernel.

FYI. Some discussion about the VFIO PCI config space saving/restoring in
live migration scenario:
https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg06964.html

thanks,
Jason


> > And another question, why don't we support MSI? I see the code only
> > handles MSIX?
> 
> Yes, needs more code for MSI.
> 
> - Steve
>   
> >>>> +
> >>>> +    } else if (vfio_pci_read_config(pdev, PCI_INTERRUPT_PIN, 1)) {
> >>>> +        vfio_intx_enable(vdev, &err);
> >>>> +        if (err) {
> >>>> +            error_report_err(err);
> >>>> +        }
> >>>> +    }
> >>>> +
> >>>> +    vdev->vbasedev.group->container->reused = false;
> >>>> +    vdev->pdev.reused = false;
> >>>> +
> >>>> +    return 0;
> >>>> +}
> >>>> +
> >>>> +static const VMStateDescription vfio_pci_vmstate = {
> >>>> +    .name = "vfio-pci",
> >>>> +    .unmigratable = 1,
> >>>> +    .mode_mask = VMS_RESTART,
> >>>> +    .version_id = 0,
> >>>> +    .minimum_version_id = 0,
> >>>> +    .post_load = vfio_pci_post_load,
> >>>> +    .fields = (VMStateField[]) {
> >>>> +        VMSTATE_MSIX(pdev, VFIOPCIDevice),
> >>>> +        VMSTATE_END_OF_LIST()
> >>>> +    }
> >>>> +};
> >>>> +
> >>>>  static void vfio_pci_dev_class_init(ObjectClass *klass, void *data)
> >>>>  {
> >>>>      DeviceClass *dc = DEVICE_CLASS(klass);
> >>>> @@ -3189,6 +3259,7 @@ static void vfio_pci_dev_class_init(ObjectClass 
> >>>> *klass, void *data)
> >>>>  
> >>>>      dc->reset = vfio_pci_reset;
> >>>>      device_class_set_props(dc, vfio_pci_dev_properties);
> >>>> +    dc->vmsd = &vfio_pci_vmstate;
> >>>>      dc->desc = "VFIO-based PCI device assignment";
> >>>>      set_bit(DEVICE_CATEGORY_MISC, dc->categories);
> >>>>      pdc->realize = vfio_realize;
> >>>> diff --git a/hw/vfio/platform.c b/hw/vfio/platform.c
> >>>> index ac2cefc..e6e1a5d 100644
> >>>> --- a/hw/vfio/platform.c
> >>>> +++ b/hw/vfio/platform.c
> >>>> @@ -592,7 +592,7 @@ static int vfio_base_device_init(VFIODevice 
> >>>> *vbasedev, Error **errp)
> >>>>              return -EBUSY;
> >>>>          }
> >>>>      }
> >>>> -    ret = vfio_get_device(group, vbasedev->name, vbasedev, errp);
> >>>> +    ret = vfio_get_device(group, vbasedev->name, vbasedev, 0, errp);
> >>>>      if (ret) {
> >>>>          vfio_put_group(group);
> >>>>          return ret;
> >>>> diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
> >>>> index bd07c86..c926a24 100644
> >>>> --- a/include/hw/pci/pci.h
> >>>> +++ b/include/hw/pci/pci.h
> >>>> @@ -358,6 +358,7 @@ struct PCIDevice {
> >>>>  
> >>>>      /* ID of standby device in net_failover pair */
> >>>>      char *failover_pair_id;
> >>>> +    bool reused;
> >>>>  };
> >>>>  
> >>>>  void pci_register_bar(PCIDevice *pci_dev, int region_num,
> >>>> diff --git a/include/hw/vfio/vfio-common.h 
> >>>> b/include/hw/vfio/vfio-common.h
> >>>> index c78f3ff..4e2a332 100644
> >>>> --- a/include/hw/vfio/vfio-common.h
> >>>> +++ b/include/hw/vfio/vfio-common.h
> >>>> @@ -73,6 +73,8 @@ typedef struct VFIOContainer {
> >>>>      unsigned iommu_type;
> >>>>      Error *error;
> >>>>      bool initialized;
> >>>> +    bool reused;
> >>>> +    int cid;
> >>>>      unsigned long pgsizes;
> >>>>      QLIST_HEAD(, VFIOGuestIOMMU) giommu_list;
> >>>>      QLIST_HEAD(, VFIOHostDMAWindow) hostwin_list;
> >>>> @@ -177,7 +179,7 @@ void vfio_reset_handler(void *opaque);
> >>>>  VFIOGroup *vfio_get_group(int groupid, AddressSpace *as, Error **errp);
> >>>>  void vfio_put_group(VFIOGroup *group);
> >>>>  int vfio_get_device(VFIOGroup *group, const char *name,
> >>>> -                    VFIODevice *vbasedev, Error **errp);
> >>>> +                    VFIODevice *vbasedev, bool *reused, Error **errp);
> >>>>  
> >>>>  extern const MemoryRegionOps vfio_region_ops;
> >>>>  typedef QLIST_HEAD(VFIOGroupList, VFIOGroup) VFIOGroupList;
> >>>> diff --git a/migration/savevm.c b/migration/savevm.c
> >>>> index 881dc13..2606cf0 100644
> >>>> --- a/migration/savevm.c
> >>>> +++ b/migration/savevm.c
> >>>> @@ -1568,7 +1568,7 @@ static int qemu_savevm_state(QEMUFile *f, 
> >>>> VMStateMode mode, Error **errp)
> >>>>          return -EINVAL;
> >>>>      }
> >>>>  
> >>>> -    if (migrate_use_block()) {
> >>>> +    if ((mode & (VMS_SNAPSHOT | VMS_MIGRATE)) && migrate_use_block()) {
> >>>>          error_setg(errp, "Block migration and snapshots are 
> >>>> incompatible");
> >>>>          return -EINVAL;
> >>>>      }
> >>>> -- 
> >>>> 1.8.3.1
> >>>>
> >>>>