Re: [Qemu-devel] [PATCH 4/5] ppc: Improve PCR bit selection in ppc_set_compat()

[Thomas Huth <thuth@redhat.com>]

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On 08.06.2016 03:12, David Gibson wrote:
> On Tue, Jun 07, 2016 at 05:39:39PM +0200, Thomas Huth wrote:
>> When using an olderr PowerISA level, all the upper compatibility

s/olderr/older/ :-/

>> bits have to be enabled, too. For example when we want to run
>> something in PowerISA 2.05 compatibility mode on POWER8, the bit
>> for 2.06 has to be set beside the bit for 2.05.
>> Additionally, to make sure that we do not set bits that are not
>> supported by the host, we apply a mask with the known-to-be-good
>> bits here, too.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
> 
> This one confused me a bit until I realised that, roughly speaking,
> bits in the PCR turn features off, rather than turning features on.
> Does that sound correct?

Right. Turning on the 2.06 bit means "disable all things that have been
added additionally in 2.07", for example. So if you turn on bit 2.05,
but not bit 2.06 (which the old code was doing), I guess you end up in a
strange state where new instructions from PowerISA 2.07 can be used, but
the instructions that have been added in ISA 2.06 are disabled.

>> ---
>>  target-ppc/translate_init.c | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/target-ppc/translate_init.c b/target-ppc/translate_init.c
>> index fa09183..ee2bc14 100644
>> --- a/target-ppc/translate_init.c
>> +++ b/target-ppc/translate_init.c
>> @@ -9519,24 +9519,29 @@ void ppc_set_compat(PowerPCCPU *cpu, uint32_t cpu_version, Error **errp)
>>  {
>>      int ret = 0;
>>      CPUPPCState *env = &cpu->env;
>> +    PowerPCCPUClass *host_pcc;
>>  
>>      cpu->cpu_version = cpu_version;
>>  
>>      switch (cpu_version) {
>>      case CPU_POWERPC_LOGICAL_2_05:
>> -        env->spr[SPR_PCR] = PCR_COMPAT_2_05;
>> +        env->spr[SPR_PCR] = PCR_TM_DIS | PCR_VSX_DIS | PCR_COMPAT_2_07 |
>> +                            PCR_COMPAT_2_06 | PCR_COMPAT_2_05;
>>          break;
>>      case CPU_POWERPC_LOGICAL_2_06:
>> -        env->spr[SPR_PCR] = PCR_COMPAT_2_06;
>> -        break;
>>      case CPU_POWERPC_LOGICAL_2_06_PLUS:
>> -        env->spr[SPR_PCR] = PCR_COMPAT_2_06;
>> +        env->spr[SPR_PCR] = PCR_TM_DIS | PCR_COMPAT_2_07 | PCR_COMPAT_2_06;
>>          break;
>>      default:
>>          env->spr[SPR_PCR] = 0;
>>          break;
>>      }
>>  
>> +    host_pcc = kvm_ppc_get_host_cpu_class();
>> +    if (host_pcc) {
>> +        env->spr[SPR_PCR] &= host_pcc->pcr_mask;
>> +    }
>> +
>>      if (kvm_enabled()) {
>>          ret = kvmppc_set_compat(cpu, cpu->cpu_version);
>>          if (ret < 0) {
> 

 Thomas



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