Re: [Qemu-devel] [PATCH v3 4/5] vfio-pci: Add FAILOVER_PRIMARY_CHANGED event to shorten downtime during failover

[Michael S. Tsirkin]

On Mon, Jan 07, 2019 at 04:17:17PM -0700, Alex Williamson wrote:
> On Mon,  7 Jan 2019 17:29:43 -0500
> Venu Busireddy <address@hidden> wrote:
> 
> > From: Si-Wei Liu <address@hidden>
> > 
> > When a VF is hotplugged into the guest, datapath switching will be
> > performed immediately, which is sub-optimal in terms of timing, and
> > could end up with substantial network downtime. One of ways to shorten
> > this downtime is to switch the datapath only after the VF is seen to get
> > enabled by guest, indicated by the bus master bit in VF's PCI config
> > space getting enabled. The FAILOVER_PRIMARY_CHANGED event is emitted
> > at that time to indicate this condition. Then management stack can kick
> > off datapath switching upon receiving the event.
> > 
> > Signed-off-by: Si-Wei Liu <address@hidden>
> > Signed-off-by: Venu Busireddy <address@hidden>
> > ---
> >  hw/vfio/pci.c | 57 
> > +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  qapi/net.json | 26 ++++++++++++++++++++++++++
> >  2 files changed, 83 insertions(+)
> 
> Why is this done at the vfio driver layer rather than the PCI core
> layer?  We write everything through using pci_default_write_config(), I
> don't see that anything here is particularly vfio specific.  Please copy
> me on any changes in hw/vfio.  Thanks,
> 
> Alex

Hmm so you are saying let's send events for each device?
I don't have a problem with this but in this case
I think I would like to see a per-device option "send events".
We don't want a ton of events in the simple default config.

> > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> > index bd83b58..adcc95a 100644
> > --- a/hw/vfio/pci.c
> > +++ b/hw/vfio/pci.c
> > @@ -34,6 +34,7 @@
> >  #include "pci.h"
> >  #include "trace.h"
> >  #include "qapi/error.h"
> > +#include "qapi/qapi-events-net.h"
> >  
> >  #define MSIX_CAP_LENGTH 12
> >  
> > @@ -42,6 +43,7 @@
> >  
> >  static void vfio_disable_interrupts(VFIOPCIDevice *vdev);
> >  static void vfio_mmap_set_enabled(VFIOPCIDevice *vdev, bool enabled);
> > +static void vfio_failover_notify(VFIOPCIDevice *vdev, bool status);
> >  
> >  /*
> >   * Disabling BAR mmaping can be slow, but toggling it around INTx can
> > @@ -1170,6 +1172,8 @@ void vfio_pci_write_config(PCIDevice *pdev,
> >  {
> >      VFIOPCIDevice *vdev = PCI_VFIO(pdev);
> >      uint32_t val_le = cpu_to_le32(val);
> > +    bool may_notify = false;
> > +    bool master_was = false;
> >  
> >      trace_vfio_pci_write_config(vdev->vbasedev.name, addr, val, len);
> >  
> > @@ -1180,6 +1184,14 @@ void vfio_pci_write_config(PCIDevice *pdev,
> >                       __func__, vdev->vbasedev.name, addr, val, len);
> >      }
> >  
> > +    /* Bus Master Enabling/Disabling */
> > +    if (pdev->failover_primary && current_cpu &&
> > +        range_covers_byte(addr, len, PCI_COMMAND)) {
> > +        master_was = !!(pci_get_word(pdev->config + PCI_COMMAND) &
> > +                        PCI_COMMAND_MASTER);
> > +        may_notify = true;
> > +    }
> > +
> >      /* MSI/MSI-X Enabling/Disabling */
> >      if (pdev->cap_present & QEMU_PCI_CAP_MSI &&
> >          ranges_overlap(addr, len, pdev->msi_cap, vdev->msi_cap_size)) {
> > @@ -1235,6 +1247,14 @@ void vfio_pci_write_config(PCIDevice *pdev,
> >          /* Write everything to QEMU to keep emulated bits correct */
> >          pci_default_write_config(pdev, addr, val, len);
> >      }
> > +
> > +    if (may_notify) {
> > +        bool master_now = !!(pci_get_word(pdev->config + PCI_COMMAND) &
> > +                             PCI_COMMAND_MASTER);
> > +        if (master_was != master_now) {
> > +            vfio_failover_notify(vdev, master_now);
> > +        }
> > +    }
> >  }
> >  
> >  /*
> > @@ -2801,6 +2821,17 @@ static void 
> > vfio_unregister_req_notifier(VFIOPCIDevice *vdev)
> >      vdev->req_enabled = false;
> >  }
> >  
> > +static void vfio_failover_notify(VFIOPCIDevice *vdev, bool status)
> > +{
> > +    PCIDevice *pdev = &vdev->pdev;
> > +    const char *n;
> > +    gchar *path;
> > +
> > +    n = pdev->qdev.id ? pdev->qdev.id : vdev->vbasedev.name;
> > +    path = object_get_canonical_path(OBJECT(vdev));
> > +    qapi_event_send_failover_primary_changed(!!n, n, path, status);
> > +}
> > +
> >  static void vfio_realize(PCIDevice *pdev, Error **errp)
> >  {
> >      VFIOPCIDevice *vdev = PCI_VFIO(pdev);
> > @@ -3109,10 +3140,36 @@ static void vfio_instance_finalize(Object *obj)
> >      vfio_put_group(group);
> >  }
> >  
> > +static void vfio_exit_failover_notify(VFIOPCIDevice *vdev)
> > +{
> > +    PCIDevice *pdev = &vdev->pdev;
> > +
> > +    /*
> > +     * Guest driver may not get the chance to disable bus mastering
> > +     * before the device object gets to be unrealized. In that event,
> > +     * send out a "disabled" notification on behalf of guest driver.
> > +     */
> > +    if (pdev->failover_primary &&
> > +        pdev->bus_master_enable_region.enabled) {
> > +        vfio_failover_notify(vdev, false);
> > +    }
> > +}
> > +
> >  static void vfio_exitfn(PCIDevice *pdev)
> >  {
> >      VFIOPCIDevice *vdev = PCI_VFIO(pdev);
> >  
> > +    /*
> > +     * During the guest reboot sequence, it is sometimes possible that
> > +     * the guest may not get sufficient time to complete the entire driver
> > +     * removal sequence, near the end of which a PCI config space write to
> > +     * disable bus mastering can be intercepted by device. In such cases,
> > +     * the FAILOVER_PRIMARY_CHANGED "disable" event will not be emitted. It
> > +     * is imperative to generate the event on the guest's behalf if the
> > +     * guest fails to make it.
> > +     */
> > +    vfio_exit_failover_notify(vdev);
> > +
> >      vfio_unregister_req_notifier(vdev);
> >      vfio_unregister_err_notifier(vdev);
> >      pci_device_set_intx_routing_notifier(&vdev->pdev, NULL);
> > diff --git a/qapi/net.json b/qapi/net.json
> > index 633ac87..a5b8d70 100644
> > --- a/qapi/net.json
> > +++ b/qapi/net.json
> > @@ -757,3 +757,29 @@
> >  ##
> >  { 'command': 'query-standby-status', 'data': { '*device': 'str' },
> >    'returns': ['StandbyStatusInfo'] }
> > +
> > +##
> > +# @FAILOVER_PRIMARY_CHANGED:
> > +#
> > +# Emitted whenever the driver of failover primary is loaded or unloaded
> > +# by the guest.
> > +#
> > +# @device: device name
> > +#
> > +# @path: device path
> > +#
> > +# @enabled: true if driver is loaded thus device is enabled in guest
> > +#
> > +# Since: 3.0
> > +#
> > +# Example:
> > +#
> > +# <- { "event": "FAILOVER_PRIMARY_CHANGED",
> > +#      "data": { "device": "vfio-0",
> > +#                "path": "/machine/peripheral/vfio-0" },
> > +#                "enabled": "true" },
> > +#      "timestamp": { "seconds": 1539935213, "microseconds": 753529 } }
> > +#
> > +##
> > +{ 'event': 'FAILOVER_PRIMARY_CHANGED',
> > +  'data': { '*device': 'str', 'path': 'str', 'enabled': 'bool' } }
> >