Re: [PATCH 2/3] xen-pciback: reconfigure also from backend watch handler

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 4/7/21 10:37 AM, Jan Beulich wrote:
> When multiple PCI devices get assigned to a guest right at boot, libxl
> incrementally populates the backend tree. The writes for the first of
> the devices trigger the backend watch. In turn xen_pcibk_setup_backend()
> will set the XenBus state to Initialised, at which point no further
> reconfigures would happen unless a device got hotplugged. Arrange for
> reconfigure to also get triggered from the backend watch handler.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> I will admit that this isn't entirely race-free (with the guest actually
> attaching in parallel), but from the looks of it such a race ought to be
> benign (not the least thanks to the mutex). Ideally the tool stack
> wouldn't write num_devs until all devices had their information
> populated. I tried doing so in libxl, but xen_pcibk_setup_backend()
> calling xenbus_dev_fatal() when not being able to read that node
> prohibits such an approach (or else libxl and driver changes would need
> to go into use in lock-step).
>
> I wonder why what is being watched isn't just the num_devs node. Right
> now the watch triggers quite frequently without anything relevant
> actually having changed (I suppose in at least some cases in response
> to writes by the backend itself).
>
> --- a/drivers/xen/xen-pciback/xenbus.c
> +++ b/drivers/xen/xen-pciback/xenbus.c
> @@ -359,7 +359,8 @@ out:
>       return err;
>  }
>  
> -static int xen_pcibk_reconfigure(struct xen_pcibk_device *pdev)
> +static int xen_pcibk_reconfigure(struct xen_pcibk_device *pdev,
> +                              enum xenbus_state state)
>  {
>       int err = 0;
>       int num_devs;
> @@ -374,8 +375,7 @@ static int xen_pcibk_reconfigure(struct
>  
>       mutex_lock(&pdev->dev_lock);
>       /* Make sure we only reconfigure once */


Is this comment still valid or should it be moved ...


> -     if (xenbus_read_driver_state(pdev->xdev->nodename) !=
> -         XenbusStateReconfiguring)
> +     if (xenbus_read_driver_state(pdev->xdev->nodename) != state)
>               goto out;
>  
>       err = xenbus_scanf(XBT_NIL, pdev->xdev->nodename, "num_devs", "%d",
> @@ -500,6 +500,9 @@ static int xen_pcibk_reconfigure(struct
>               }
>       }
>  
> +     if (state != XenbusStateReconfiguring)
> +             goto out;
> +


... here?


>       err = xenbus_switch_state(pdev->xdev, XenbusStateReconfigured);
>       if (err) {
>               xenbus_dev_fatal(pdev->xdev, err,
> @@ -525,7 +528,7 @@ static void xen_pcibk_frontend_changed(s
>               break;
>  
>       case XenbusStateReconfiguring:
> -             xen_pcibk_reconfigure(pdev);
> +             xen_pcibk_reconfigure(pdev, XenbusStateReconfiguring);
>               break;
>  
>       case XenbusStateConnected:
> @@ -664,6 +667,10 @@ static void xen_pcibk_be_watch(struct xe
>               xen_pcibk_setup_backend(pdev);
>               break;
>  
> +     case XenbusStateInitialised:
> +             xen_pcibk_reconfigure(pdev, XenbusStateInitialised);


Could you add a comment here that this is needed when multiple devices are 
added?


It also looks a bit odd that adding a device is now viewed as a 
reconfiguration. It seems to me that xen_pcibk_setup_backend() and 
xen_pcibk_reconfigure() really should be merged --- initialization code for 
both looks pretty much the same.


-boris