Re: [PATCH 3/3] spapr: Migrate SpaprDrc::unplug_requested

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Feb 03, 2020 at 11:36:22PM +0100, Greg Kurz wrote:
> Hot unplugging a device is an asynchronous operation. If the guest is
> migrated after the event was sent but before it could release the
> device with RTAS, the destination QEMU doesn't know about the pending
> unplug operation and doesn't actually remove the device when the guest
> finally releases it. The device
> 
> Migrate SpaprDrc::unplug_requested to fix the inconsistency. This is
> done with a subsection that is only sent if an unplug request is
> pending. This allows to preserve migration with older guests in the
> case of a pending hotplug request. This will cause migration to fail
> if the destination can't handle the subsection, but this is better
> than ending with an inconsistency.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  hw/ppc/spapr_drc.c |   27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
> index d512ac6e1e7f..6f5cab70fc6b 100644
> --- a/hw/ppc/spapr_drc.c
> +++ b/hw/ppc/spapr_drc.c
> @@ -455,6 +455,22 @@ void spapr_drc_reset(SpaprDrc *drc)
>      }
>  }
>  
> +static bool spapr_drc_unplug_requested_needed(void *opaque)
> +{
> +    return spapr_drc_unplug_requested(opaque);
> +}
> +
> +static const VMStateDescription vmstate_spapr_drc_unplug_requested = {
> +    .name = "spapr_drc/unplug_requested",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .needed = spapr_drc_unplug_requested_needed,
> +    .fields  = (VMStateField []) {
> +        VMSTATE_BOOL(unplug_requested, SpaprDrc),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
>  static bool spapr_drc_needed(void *opaque)
>  {
>      SpaprDrc *drc = (SpaprDrc *)opaque;
> @@ -467,8 +483,11 @@ static bool spapr_drc_needed(void *opaque)
>      /*
>       * We need to migrate the state if it's not equal to the expected
>       * long-term state, which is the same as the coldplugged initial
> -     * state */
> -    return !spapr_drc_device_ready(drc);
> +     * state, or if an unplug request is pending.
> +     */
> +    return
> +        spapr_drc_unplug_requested_needed(drc) ||
> +        !spapr_drc_device_ready(drc);

Hrm.  You start the series by splitting spapr_drc_device_ready() from
spapr_drc_needed().  But at this point, I'm pretty sure you've now got
all the callers of spapr_drc_device_ready() doing equivalent logic
about them, so they might as well be one function again.  That seems
pretty roundabout.

I don't think the rationale for not using the drc_ready function from
the CAS path really makes sense.  It's not just an accident that those
use the same logic - in both cases what we're testing is "Is the DRC
in a state other than that of a default cold-plugged device?".

Changing the name might be sensible, but I still think we want a
common function for the two cases.

>  }
>  
>  static const VMStateDescription vmstate_spapr_drc = {
> @@ -479,6 +498,10 @@ static const VMStateDescription vmstate_spapr_drc = {
>      .fields  = (VMStateField []) {
>          VMSTATE_UINT32(state, SpaprDrc),
>          VMSTATE_END_OF_LIST()
> +    },
> +    .subsections = (const VMStateDescription * []) {
> +        &vmstate_spapr_drc_unplug_requested,
> +        NULL
>      }
>  };
>  
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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