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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon,  7 Jan 2019 17:29:43 -0500
Venu Busireddy <venu.busireddy@oracle.com> wrote:

> From: Si-Wei Liu <si-wei.liu@oracle.com>
> 
> 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 <si-wei.liu@oracle.com>
> Signed-off-by: Venu Busireddy <venu.busireddy@oracle.com>
> ---
>  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

> 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' } }
>