Re: [PULL 25/72] Adds migration support for Branch History Rolling Buffer (BHRB) internal state.

[Fabiano Rosas]

Nicholas Piggin <npiggin@gmail.com> writes:

> From: Glenn Miles <milesg@linux.vnet.ibm.com>
>
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Glenn Miles <milesg@linux.vnet.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  target/ppc/machine.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> index 6b6c31d903..731dd8df35 100644
> --- a/target/ppc/machine.c
> +++ b/target/ppc/machine.c
> @@ -711,6 +711,26 @@ static const VMStateDescription vmstate_reservation = {
>      }
>  };
>  
> +#ifdef TARGET_PPC64
> +static bool bhrb_needed(void *opaque)
> +{
> +    PowerPCCPU *cpu = opaque;
> +    return (cpu->env.flags & POWERPC_FLAG_BHRB) != 0;
> +}
> +
> +static const VMStateDescription vmstate_bhrb = {
> +    .name = "cpu/bhrb",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .needed = bhrb_needed,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_UINTTL(env.bhrb_offset, PowerPCCPU),
> +        VMSTATE_UINT64_ARRAY(env.bhrb, PowerPCCPU, BHRB_MAX_NUM_ENTRIES),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +#endif
> +
>  const VMStateDescription vmstate_ppc_cpu = {
>      .name = "cpu",
>      .version_id = 5,
> @@ -756,6 +776,7 @@ const VMStateDescription vmstate_ppc_cpu = {
>  #ifdef TARGET_PPC64
>          &vmstate_tm,
>          &vmstate_slb,
> +        &vmstate_bhrb,

Running some tests now that Nick re-enabled ppc for migration tests, I
see that this new state breaks backward migrations:

$ QTEST_TRACE="vmstate_*" \
  QTEST_QEMU_BINARY_DST=../build-previous/qemu-system-ppc64 \
  QTEST_QEMU_BINARY=./qemu-system-ppc64 \
  ./tests/qtest/migration-test -p /ppc64/migration/precopy/tcp/plain
...
vmstate_load_state_field cpu/slb:env.slb
vmstate_n_elems env.slb: 64
vmstate_subsection_load cpu/slb
vmstate_subsection_load_bad cpu/slb: cpu/bhrb/(prefix)
vmstate_load_state_end cpu/slb end/0
vmstate_subsection_load_bad cpu: cpu/bhrb/(lookup)
qemu-system-ppc64: error while loading state for instance 0x0 of device 'cpu'
vmstate_downtime_checkpoint dst-precopy-loadvm-completed
qemu-system-ppc64: load of migration failed: No such file or directory

If you want to support backwards migration, then this needs to be
fixed. Otherwise we can ignore it.