Re: [Qemu-devel] [PATCH v2] spapr: manage hotplugged devices while the VM is not started

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

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On Wed, May 31, 2017 at 11:25:16AM +0200, Laurent Vivier wrote:
> For QEMU, a hotlugged device is a device added using the HMP/QMP
> interface.
> For SPAPR, a hotplugged device is a device added while the
> machine is running. In this case QEMU doesn't update internal
> state but relies on the OS for this part
> 
> In the case of migration, when we (libvirt) hotplug a device
> on the source guest, we (libvirt) generally hotplug the same
> device on the destination guest. But in this case, the machine
> is stopped (RUN_STATE_INMIGRATE) and QEMU must not expect
> the OS will manage it as an hotplugged device as it will
> be "imported" by the migration.
> 
> This patch changes the meaning of "hotplugged" in spapr.c
> to manage a QEMU hotplugged device like a "coldplugged" one
> when the machine is awaiting an incoming migration.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> v2:
> - don't replace dev->hotplugged to test if CPU hotplug
>   is supported.

So, this addresses the specific points mentioned on the last version,
but not the wider query: what exactly in the previous behaviour was
causing problems.

On a related note, having better understood the DRC code while writing
my cleanups, I'm reconsidering your earlier suggestion of simply
disabling hotplugs until after CAS.

I rejected that approach before because I was assuming the problem was
due to hotplug events being lost, and I was confident that CAS marked
a transition from losing to not-losing events (other than by
accident).  However, later discussions have shown that the problem is
more to do with device tree updates being essentially duplicated
between the DT given to the guest during CAS and further updates from
hotplug events.

On that new understanding CAS does seem like a logical watershed,
since it's the point at which the guest gets the final version of the
"cold plugged" device tree.  I still have some concerns about the
details, but the basic approach seems sound.

Sorry for the misdirection.

> 
>  hw/ppc/spapr.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index ab3aab1..f0c543c 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2521,6 +2521,12 @@ static void spapr_nmi(NMIState *n, int cpu_index, Error **errp)
>      }
>  }
>  
> +static bool spapr_coldplugged(DeviceState *dev)
> +{
> +    return runstate_check(RUN_STATE_INMIGRATE) ||
> +           !dev->hotplugged;
> +}
> +
>  static void spapr_add_lmbs(DeviceState *dev, uint64_t addr_start, uint64_t size,
>                             uint32_t node, bool dedicated_hp_event_source,
>                             Error **errp)
> @@ -2531,6 +2537,7 @@ static void spapr_add_lmbs(DeviceState *dev, uint64_t addr_start, uint64_t size,
>      int i, fdt_offset, fdt_size;
>      void *fdt;
>      uint64_t addr = addr_start;
> +    bool coldplugged = spapr_coldplugged(dev);
>  
>      for (i = 0; i < nr_lmbs; i++) {
>          drc = spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_LMB,
> @@ -2542,9 +2549,9 @@ static void spapr_add_lmbs(DeviceState *dev, uint64_t addr_start, uint64_t size,
>                                                  SPAPR_MEMORY_BLOCK_SIZE);
>  
>          drck = SPAPR_DR_CONNECTOR_GET_CLASS(drc);
> -        drck->attach(drc, dev, fdt, fdt_offset, !dev->hotplugged, errp);
> +        drck->attach(drc, dev, fdt, fdt_offset, coldplugged, errp);
>          addr += SPAPR_MEMORY_BLOCK_SIZE;
> -        if (!dev->hotplugged) {
> +        if (coldplugged) {
>              /* guests expect coldplugged LMBs to be pre-allocated */
>              drck->set_allocation_state(drc, SPAPR_DR_ALLOCATION_STATE_USABLE);
>              drck->set_isolation_state(drc, SPAPR_DR_ISOLATION_STATE_UNISOLATED);
> @@ -2553,7 +2560,7 @@ static void spapr_add_lmbs(DeviceState *dev, uint64_t addr_start, uint64_t size,
>      /* send hotplug notification to the
>       * guest only in case of hotplugged memory
>       */
> -    if (dev->hotplugged) {
> +    if (!coldplugged) {
>          if (dedicated_hp_event_source) {
>              drc = spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_LMB,
>                      addr_start / SPAPR_MEMORY_BLOCK_SIZE);
> @@ -2867,6 +2874,7 @@ static void spapr_core_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>      int smt = kvmppc_smt_threads();
>      CPUArchId *core_slot;
>      int index;
> +    bool coldplugged = spapr_coldplugged(dev);
>  
>      core_slot = spapr_find_cpu_slot(MACHINE(hotplug_dev), cc->core_id, &index);
>      if (!core_slot) {
> @@ -2888,7 +2896,7 @@ static void spapr_core_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>  
>      if (drc) {
>          sPAPRDRConnectorClass *drck = SPAPR_DR_CONNECTOR_GET_CLASS(drc);
> -        drck->attach(drc, dev, fdt, fdt_offset, !dev->hotplugged, &local_err);
> +        drck->attach(drc, dev, fdt, fdt_offset, coldplugged, &local_err);
>          if (local_err) {
>              g_free(fdt);
>              error_propagate(errp, local_err);
> @@ -2896,7 +2904,7 @@ static void spapr_core_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>          }
>      }
>  
> -    if (dev->hotplugged) {
> +    if (!coldplugged) {
>          /*
>           * Send hotplug notification interrupt to the guest only in case
>           * of hotplugged CPUs.

-- 
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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