Re: [Qemu-devel] [PATCH] spapr: fix memory hot-unplugging

[Laurent Vivier <lvivier@redhat.com>]

Adding cc: Bharata.

On 27/03/2017 15:22, Laurent Vivier wrote:
> If, once the kernel has booted, we try to remove a memory
> hotplugged while the kernel was not started, QEMU crashes on
> an assert:
> 
>     qemu-system-ppc64: hw/virtio/vhost.c:651:
>                        vhost_commit: Assertion `r >= 0' failed.
>     ...
>     #4  in vhost_commit
>     #5  in memory_region_transaction_commit
>     #6  in pc_dimm_memory_unplug
>     #7  in spapr_memory_unplug
>     #8  spapr_machine_device_unplug
>     #9  in hotplug_handler_unplug
>     #10 in spapr_lmb_release
>     #11 in detach
>     #12 in set_allocation_state
>     #13 in rtas_set_indicator
>     ...
> 
> If we take a closer look to the guest kernel log, we can see when
> we try to unplug the memory:
> 
>     pseries-hotplug-mem: Attempting to hot-add 4 LMB(s)
> 
> What happens:
> 
>     1- The kernel has ignored the memory hotplug event because
>        it was not started when it was generated.
> 
>     2- When we hot-unplug the memory,
>        QEMU starts to remove the memory,
>             generates an hot-unplug event,
>         and signals the kernel of the incoming new event
> 
>     3- as the kernel is started, on the QEMU signal, it reads
>        the event list, decodes the hotplug event and tries to
>        finish the hotplugging.
> 
>     4- QEMU receives the hotplug notification while it
>        is trying to hot-unplug the memory. This moves the memory
>        DRC to an invalid state
> 
> This patch prevents this by not allowing to set the allocation
> state to USABLE while the DRC is awaiting release and allocation.
> 
> RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1432382
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/ppc/spapr_drc.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
> index 150f6bf..377ea65 100644
> --- a/hw/ppc/spapr_drc.c
> +++ b/hw/ppc/spapr_drc.c
> @@ -135,6 +135,12 @@ static uint32_t set_allocation_state(sPAPRDRConnector *drc,
>          if (!drc->dev) {
>              return RTAS_OUT_NO_SUCH_INDICATOR;
>          }
> +        if (drc->awaiting_release && drc->awaiting_allocation) {
> +            /* kernel is acknowledging a previous hotplug event
> +             * while we are already removing it.
> +             */

I'm wondering if I should set drc->awaiting_allocation to false here?

> +            return RTAS_OUT_NO_SUCH_INDICATOR;
> +        }
>      }
>  
>      if (drc->type != SPAPR_DR_CONNECTOR_TYPE_PCI) {
> 

Thanks,
Laurent